For my configuration using this phone,
I was able to twist connect the two data wires together, which then my phone then stopped the naggy slow charge message.
I am aware that Apple uses a resistor across the data lines for their phones.
Link provided in comments was supplementary to my goal.
If all else fails, and if you can reliably start the thing up with peripherals unplugged and then plug them in later, then you could intentionally stage the turn-on of the peripherals. Just have them on timed relays (or relays controlled from a microprocessor) that sequence them on in a way that doesn't challenge the power supply too much.
Regarding the electrical interface you need stereo sound so try to use the CD inputs.
Get a cheap pair of headphones or a 3.5 mm stereo jack extension lead and cut the headphones off. Connect the tip to C19, ring to C20 and sleeve to C18. (Use your multimeter continuity tester to identify the cores.) Try it out and if it ...
I figured out the turn signal wiring! The headlight simply required two B+ inputs.
The Grand Touring and Signature headlights require 2 constant 12v inputs for each side of the front turn signals which the Sport and Touring harness is missing.
What I did to test this theory was wire an always on 12v to the 2 wires- white/black wire and blue wire on the GT/...
In automotive charging systems the alternator output current is controlled by adjusting the alternator field current. The alternator regulator senses the battery voltage and adjusts the field current to keep that voltage at a suitable level to keep the batteries charged.