NOTE: Tutorial (of sorts) alert.
The hex Schmitt trigger based flasher circuit towards the end of this answer is as much an example of what can be done with a simple "digital" IC by using somewhat unusual methods. A RTL OR gate is used, a diode OR gate and a few other strange techniques.
Once it is seen what can be done in this manner 'all sorts' of useful circuits can be implemented at low cost. The usual downside is a higher parts count of low cost parts and some interim head scratching.
You can replace the flasher relay with a P Channel MOSFET with gate driven by Q1 and a resistor from gate to source (to allow oscillator to work as before.) They show what appears to be 400 Ohms across coil implying this is in parallel with coil resistance. If so then actual value will be lower.
MOSFET source to terminal 49.
MOSFET drain to terminal 49A.
MOSFET gate to Q1 directly.
400 Ohms or less gate to 49.
Load sense relay operates when current is high enough and shorts 200 Ohm Q2 collector resistor. Effect obscure without further head scratching but affects time constant of Q1 Q2 100 uF oscillator feedback loop.
The load sense relay could be replaced by a comparator and FET BUT as long as about 0.6 Volt drop was acceptable across the controller, you should be able to use simply a small transistor with a sense resistor across its base-emitter. If I_load caused > 0.6V drop in the sense resistor the transistor would turn on and you could use it to operate another small and cheap bipolar transistor to short out the 200 Ohm resistor instead of using relay contacts. The resultant circuit would use no relays and only one MOSFET to provide flashing action.
Below is an example only circuit based on a modified version of the one you provided. Q3 is probably about right - the 400 Ohm resistor that was across the relay coil will need to be lower. valued. If this was the coil resistance then an approximately 400 Ohm resistor will be needed. Q4 sensing probably would work OK but the drive of Q5 is inadequate as shown. A little more thought would produce a working result.
However, if you can provide a good specification of how it should behave then a completely new circuit may be more satisfactory. A 555 or 74C14 or LM358 or LM339 based circuit would probably work better.
Questions from my comments above:
Are there constraints on cost or using an IC or using a microcontroller or ...?
This can be done easily with a '555 or with a 74C14 (used as a timer circuit) or in various other ways. Would a cheap IC be acceptable?
I assume 31=ground, 49=battery 49a = load. Yes?
Are you trying to replace an existing part.
Is this intended to be a one off, or for a few devices, or are many to be made?
Hex Schmitt trigger example (74C14 40106 4584 ... ):
This is as much an example of what can be achieved with a single package of 6 x Schmitt triggered inverters.
A solution along these lines could be made to work well (with some playing probably being needed to deal with inrush current when 1 bulb is present (see text) and tuning of Rs to allow reliable high/low current differentiation.
BUT I'd probably use a microcontroller if at all possible.
BUT if you understand what this circuit does or tried to do then writing a microcontroller program would be much easier.
Below is a circuit diagram (that Olin may not approve of) intended only as an example.
This is "out of my head" and untried but may even work :-).
IC1 - hex Schmitt triggered inverters.
MUST be suitable for automotive supply voltages.
If used on 12V then 15 or 18V rating preferred.
Note that eg 74HC14 is usually 6V max Vdd.
This Digikey search provides possibly suitable parts.
20V - STM 40106
18V TI 40106
18V ON-Semi 4584
Note that some hex Schmitt trigger inverters have lower hysteresis ranges than normal so oscillators run faster with the same RC values.
Rz, Z1, Cz at top left provide a Vdd supply for the IC slightly below Vbattery min to help keep flash rate somewhat stable.
There are two flash oscillators, fast and slow. While it would be possible to alter the rate of a single oscillator, because there are 6 inverters available, this is an easy approach and allows independent setting of fast and slow rate. It has good and bad points.
R1 C1 and one inverter form an oscillator and R2 C2 and another inverter form another. This is a very standard circuit when using a Schmitt triggered gate. It will not work with a non Schmitt-triggered gate.
The heart of the current sense for slow/fast swapping is RS and Q1. When bulb current causes a drop across Rs of more than about 0.6V Q1 turns on. If current is too low to cause 0.6V drop OR if lamp is off then Q1 will be turned off.
So as R = V / I, for Ihigh Rs > 0.6/Ihigh and for Ilow Rs < 0.6/Ilow. A value of Rs should be able to be found that allows Q1 to turn on only when the high current load (2+ bulbs) is being powered.
When Q1 turns on point B (circled) is pulled high (to about Vin). As Q1 cycles on and off with flashing capacitor Cd bold point B high during off halves of flasher cycles. Rd discharges Cd when Vin is removed in preparation for next poweron or to allow rapid response if a bulb fails.
Diodes D1 and D2 are oscillator gating switches. When the voltage to the left of these diodes is low the diodes conduct, Capacitor C1 or C2 is discharged to low and the related oscillator is disabled. The oscillator gate outputs are high when disabled (as input is clamped low) and as will be seen, the gate cannot then drive the output.
SLOW FLASH / 2 BULBS:
Consider the 2 bulbs condition after a few flashes.
Q1 turns on on each flash.
Cd charges high so point C is low so the upper (fast)oscillator is disbaled and point E is high. Diode D3 is blocked.
BUT As B is high, point D is also high so D2 is reverse biased so slow oscillator with R2 C2 is enabled and runs. When point F goes low during oscillation D4 conducts and turns on MOSFET Q2 to drive bulbs. This provides an on pulse for Q1 which keeps point B high and holds system in slow flash mode. The slow oscillator turns Q2 on when output is low and the FET is truned off by Rg when oscillator output is high.
D3, D4 and Rg + Q2 form a DTL OR gate. (Or NOR gate depending on your perspective).
FAST FLASH / BULB Failure:
If a bulb fails Q1 never turns on as drop across Rs is too low (by design).
Point B is taken low by Rd discharging Cd.
Point D is now low, disabling the slow oscillator.
Point C is high, enabling the fast oscillator.
Operation is as above but now with the fast oscillator running.
Diode DP3 (top left) provides reverse battery protection IF WANTED.
Diode DP1 provides protection against inductive loads. This should not be necessary with lmp lods but is shown for completeness.
Diode DP2 is an alternative of sorts to DP1 - if excessive voltage is supplied to the output it will shunt it to supply if DP3 is not used. This is optional and not liable to be wanted.
Rs needs to be wattage rated high enough to take flasher current. As Vrs <=~ 0.6V usually
the Pr_Rs = V x I = 0.6 x Iload. For 2x 20 W bulbs Imax ~= 40/12 = 3.33 A. Inrush current will be higher than this. Current is present only 1/2 the time. Allow for say 5A x 0.6V = 3W x 50% on = 1.5 W. A 5 Watt resistor is cheap and probably sensible.
Inrush current MAY cause problems due to the peak current at turn on even with a single bulb and you may have to add a resistor between Q1 and Cs to slow the charge enough to avoid inrush charging when a bulb is blown. There are other ways to handle this but a simple RC delay is probably good enough.
See IC data sheets for oscillator frequency but frequency of VERY ROUGHLY 1/(R x C) applies.
Q2 should have enough current rating to handle inrush currents when bulbs are cold.
If IC1 (gates) Vdd is > about 0.6V below Vin then FET gate will be driven slightly positive when off. This is not a problem as long as drive voltages are understood and designed.
Using Asian (China) prices - which should be close to what can be obtained in India in volume:
1 Rupee ~~~= 2 cents US.
IC1 - 10 cents or less. (5 Rupee)
Q2 - 6 cents (3 Rupee)
Z1, 1,D1,2,3,4 1N4148 or similar < 1 Rupee
Other components are "glue" minimal price.
Say 30c - 40c (15-20 Rupee) all up for components ?
Microcontroller = uC.
Needs programming per item and needs program written.
An Arduino would be able to do this at a cost per uC = IC cost.
Main advantage of Arduino would be ease of programming for people with minimal uC experience. (Olin will not like this suggestion!)
If the uC has a low voltage comparator it should be able to be used for Rs sensing.
If not (VERY low cost IC) then the arrangement shown for Q1/Rs in above circuit would work.
A basic power supply to denoise Vdd is necessary.
Rz, Z1, Cz from above cct may be enough. A 2 stage zener power supply is cheap and extremely good at noise reduction (repeat as shown with 2nd stage fed by first.
If Vdd is well below Vin then an N Channel MOSFET on ground side of flasher would be useful BUT needs a connection change to standard wiring. So an extra transistor (1 cent or so) to translate drive to MOSFET gate MAY be needed.
"Hanging" uC supply off upper power rail works well and allows direct P Chanllel MOSFET drive. ie Vdd = Vin. uC_ground = say Vin - 8V.
A uC with good anti-brownout and undervoltage reset is a very good idea.
Almost any uC will do. $US0.20 or less in volume.
PIC0F204 - $US).38 / 3000 at Digikey - no doubt cheaper when arms are bent privately, is very suitable.
It has a comparator with 0.6V bandgap reference. Put uC in lower leg with say 5V supply and use one transistor to drive high side P Channel MOSFET. Olin would approve :-).
3 terminal regulator can be used for PSU but 2 stage zener adequate and probably cheaper and maybe better.
Questions? Ask ...