Where I would start is by measuring the voltages on the bulb's socket, but to be valid, that will need to be done with the bulb plugged in.
You must find 12 volts on one wire and 0 volts on the other wire. If you have a double-filament bulb, don't worry about the third wire. What we're concerned with is a 12 volt and 0 volt wire.
I suspect you're going to find around 3 â€ 6 volts on one wire and 12 volts on another one. If you do find 12 volts, ground the other one. If that makes the bulb get full brightness, that ground wire is suspect.
Do you have a single bulb with the high and low beam filaments in it or do you have two separate bulbs, one high beam and one low beam?
I'm having internet trouble tonight and can't get into the wiring diagrams. Instead, tell me if you have one or two bulbs on each side, and what you have for voltages and the wire colors they correspond to. When I can look at a diagram, I'll be able to figure out where to go next. You may need to unplug the Body Computer, then the headlights should work like on any other car without daytime running lamps. That will make diagnosis easier.
Saturday, March 26th, 2016 AT 9:21 PM