One wire is for the brake/directional filament and the other wire is for the tail light. The fixture should be grounded when it is attached to the bracket. To test which is which (the brown wire is USUALLY the ail light) make sure the fixture is grounded and touch a hot wire to each of the wires coming from the fixture. Watch to see which wire lights up the brighter filament and this is the brake/directional light. The dimmer filament if the tail light. Brakes lights and directional lights are both the same filament. Wen the brakes are applied, both lights are activated. When a direction light is turned on, it simply flashes the brake light filament.
On a two wire light you have a dual element bulb, the brake lights and turn signal work on one wire and the tail lights on the other one. On a dual element bulb on element is brighter than the other, the brighter one is for turn signal and brake light.
The trailer is a two wire system. The truck is a three wire system so you'll have to buy the adapter to send the three wire signals to the two wire trailer. To be quite honest, there are wiring harnesses that will plug directly into your wiring system without cutting or splicing your original harness and is the easiest way to convert the system. You can pick up the harness or adapter at most U Haul rental facilities. Either that, or you'll have to buy three wire lights for the trailer.
Usually on a two wire vehicle system right stop and turn use the same wire going to the right trailer light. Same with the left. And tail lights work the same way.
However since you have a three wire system with the turn signals being amber and not part of the brake lights you'll have to add another light to each side of the trailer to make it work. As an example:Right side, the trailer would need a signal wire going from the truck to operate right turn signal, right brake light, right running light. Three wire (because of the amber turn signal).
You could always run the stop and turn to the existing trailer light, and add a cheaper light for the running lights.
Hope that explained it well enough. If you have any further questions, just post a comment and I'll try to help.
6 pin socket
6 pin plug
7 pin
socket
7 pin plug
5 pin socket
5 pin plug
7 pin socket
7 pin plug
7 pin socket
7 pin plug
7 pin socket
7 pin plug
12 pin socket
12 pin plug
small 6 pin round plug
& socket
1 tail, clearance - brown
2 left hand turn - yellow
3 right hand turn - green
4 stop - red
5 electric brakes - blue
6 earth - white
small 7 pin
round plug & socket
1 left hand turn - yellow
2 reverse -
black
3 earth -
white
4 right hand turn - green
5 electric brakes - blue
6 stop - red
7 tail, clearance, side
markers - brown
round socket
large 5 pin round plug & socket
2 left hand turn - yellow
3 earth -
white
5 right hand turn -
green
6 stop - red
7 tail, clearance, side
markers - brown
large 7 pin
round plug & socket
1 left hand turn - yellow
2 reverse -
black
3 earth - white
4 right hand turn - green
5 electric brakes - blue
6 stop - red
7 tail, clearance, side
markers - brown
7 pin round
heavy duty plug & socket
1 earth -
white
2 left hand rear , clearance and marker - black
3 left hand turn - yellow
4 stop - red
5 right hand turn -
green
6 right hand rear , clearance and marker - brown
7 reverse/auxilary -
blue
7
pin flat plug & socket
1 left hand turn - yellow
2 reverse -
black
3 earth - white
4 right hand turn - green
5 electric brakes - blue
6 stop - red
7 tail, clearance, side
markers - brown
flat socket
12 pin flat
plug & socket
1 left hand turn - yellow
2 reverse -
black
3 earth - white
4 right hand turn - green
5 electric brakes - blue
6 stop - red
7 tail, clearance, side
markers - brown
8 battery supply -
orange
9 auxiliaries - pink
10 earth - white
11 rear fog lamp - grey
12 auxiliaries - violet
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