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I'm trying to make my fog lights work with the parking lights, so I have one continuous strip of light. While I'm handy enough to craft it, designing the circuit so that I retain proper factory functionality is beyond my skills. I'll try my best to explain what I need, please go easy on me if I mess up some terms.

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  1. The rear parking lights consist of one 21/5W (leftmost, also brake light) and one 5W bulb (center). The fog light uses а 21W bulb (rightmost, in the trunk partition of the taillight).
  2. Since the fog light is driver-side only, I'll add another 21W bulb to the passenger side.
  3. I'll run a wire from the driver-side 5W parking light, into the trunk, through the components that you kindly suggest I use, and finally to both fog light bulbs.
  4. There needs to be a resistor somewhere along the line, and the bulbs need to be wired in paralell, due to the bulb holder design.

The result of this mess - when I switch the parking lights on, the fog lights also light up - using 5W of power each (hence the resistor), so they are exactly as bright as the other two bulbs.

Here come the tricky parts:

  • I have no clue how to calculate the proper resistor needed to power two 21W bulbs, wired in parallel, at 5W
  • Maintaining the fog light functionality - the driver side bulb should be able to draw full power from the factory wire when needed
  • I would like current to only go one way - no current should go from the fog light to the parking light, or from the parking light to the headlight switch module
  • The original fog light circuit needs to be the default - the CANBUS system checks the bulb at startup and I refuse to live with a "bulb out" warning

I would like to use simple components - relays, diodes and so on.

A million thanks!

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    \$\begingroup\$ "so they are exactly as bright as the other two bulbs." - May not work. A 21W bulb may or may not (probably not) be as bright or of similar color as a 5W bulb at 5W each. \$\endgroup\$ – JimmyB Apr 24 '18 at 18:45
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    \$\begingroup\$ @jsotola Not sure we're talking about the same thing here, because it may vary by language and jurisdiction. But there are countries where "fog lights" in the front are plain white lights which may legally only be turned on in rain/fog. And there are also red fog taillights brighter than regular back lights and fitted on one or both sides also only for use in rain/fog. \$\endgroup\$ – JimmyB Apr 24 '18 at 18:48
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    \$\begingroup\$ @AndrewMorton He may not be (street) legally modifying the lights on his vehicle at all because it may void his car's type approval. \$\endgroup\$ – JimmyB Apr 24 '18 at 18:50
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    \$\begingroup\$ @darksky 1) Then what do you call the bright, red extra lights in the back you may turn on in fog? 2) We're not talking LED but incandescent here. \$\endgroup\$ – JimmyB Apr 24 '18 at 19:46
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    \$\begingroup\$ Here in Europe there are two types of fog lights: White or yellow at the front and red at the back. The red rear one is obligatory. The front ones are optional but there are rules about angle and brightness. I can't for the world see how any authority would allow two white rear fog lights! They would panic any driver if they appear in front of them and would (not could!) cause fatal swerving! \$\endgroup\$ – Oldfart Apr 24 '18 at 20:07
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schematic

simulate this circuit – Schematic created using CircuitLab

A basic circuit which ought to work would be as above. The diodes should be rated at least 2A - the 1N5400 is 3A. The tricky bit is going to be the resistors. If you put 5W through a 21W lamp, it will glow feebly. You would need some trial-and-error to find what actually works. The resistors need to be high power wire-wound ones.

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  • \$\begingroup\$ Simon, thank you for taking the time! I have two questions though. 1) Why are there two 5W bulbs? If that is meant to be the passenger side bulb, that one has its own wiring - the driver and the passenger side are completely separate. 2) Could I use a single resistor for both 21W bulbs? Basically remove R1 and have the wire going to D1 branch off after R2? \$\endgroup\$ – J. Doe Apr 25 '18 at 6:20
  • \$\begingroup\$ Oh, and a third question - if I understand correctly, diodes still let just a tiny bit of current the opposite way. Should I be worried about this, or is it negligable? \$\endgroup\$ – J. Doe Apr 25 '18 at 6:25
  • \$\begingroup\$ @J.Doe I was thinking of the left and right bulbs. You can forget about the second one. I will try updating the diagram. \$\endgroup\$ – Simon B Apr 25 '18 at 13:27
  • \$\begingroup\$ @J.Doe I did the two 21W lamps with separate resistors to avoid doubling the load on the fog light circuit. If that circuit can handle two fog lamps, then that would be simpler. \$\endgroup\$ – Simon B Apr 25 '18 at 13:29
  • \$\begingroup\$ I did this yesterday and it worked like a charm. The store only had 1N5402 diodes and up, and I got a 25W 10 ohm adjustable resistor, so all components exceed specs, giving me peace of mind. The resistor I fixed at around 65%, so I guess 6-7 ohms results in the correct brightness. Thanks, Simon! \$\endgroup\$ – J. Doe Apr 30 '18 at 9:18
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Let me try to draw what I think you want to do.

This is the starting point:

schematic

simulate this circuit – Schematic created using CircuitLab

And this is about what you want:

schematic

simulate this circuit

Add one diode between each R and its lamp and you can use a SPST relay instead which may be acquired more easily/cheaply.

As Simon B said, the resistor values for matching brightness need to be figured out by experiment; maybe start at 50 Ohm and reduce in steps of 5-10 Ohm until you're satisfied. As to their power rating, they should be on the order of the lamps, i.e. 20W should be pretty safe. Note that the resistors may get hot while running, so allow for some space and means to transfer the heat away.

Also, the bulb out check might get confused because of the extra current the relay draws from the parking light. It may not detect the 'overcurrent' but may fail to detect a broken bulb because some current still flows through the relay.

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    \$\begingroup\$ JimmyB, thank you for the answer! I chose to go with Simon's solution for two reasons: 1) There was the possibility that the canbus check energises the coil for a brief moment at engine startup, making the bulbs flash briefly. 2) I was going to be constantly concerned about the relay taking a beating from me just closing the trunk. \$\endgroup\$ – J. Doe Apr 30 '18 at 9:24

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