Wiring Help?

[amo43876221]

I have a 2004-2006 chevy C8500 dump truck. Now the truck itself is stock but the dump body is a samco Inc. body. I am trying to hook up a trailer brake controler. I need to hook up four wires for the controler to work, I know where three of them go already I need help on the fourth wire which is the stoplight wire. Now above the brake pedal is a little white box with a wiring harness attached to it with about 6 wires coming from it. Now what this box does is when you hit the brake it activates the lights ( the reason I know this is because we where having trouble with the brake lights on the truck and when we moved this box the lights would work again). Now can anyone tell me which one is the actual stoplight wire on that harness? Or are they all stoplight wires?
Thanks for the Help

[Kranky]

You'll have to find the wire that is "live" when the brakes are applied. That is the wire you need to connect to the trailer brake controller.

To find it, use a circuit tester (test light) you may have to use a razor blade and "nick" the insulation on the wires you're testing so that the test light probe can get thru the insulation and contact the copper wire.Check the wires at the stop light switch with the brakes off, anything live at this time is NOT the wire you want. Depress the brake pedal so that the stop lights are on and check the wires now. A wire which was not live during the first test, but is live now is the wire you want to connect to the brake controller.

After making your connection, be sure to tape up any damage to the wiring caused by the test probe.

Caution: Make sure you do not connect the brake controller to one of the wires downstream from the turn signal switch which power the stop & turn signals. This will cause no trailer braking or intermittent trailer braking when the turn signal on that side is activated.

The controller must be connected directly at the stop light switch.

All the trailer brake controllers I've ever installed have 4 wires:A main power wire which must be connected to the positive terminal of the battery, A ground wire which must be connected to the negative terminal of the battery, A brake wire which must be connected to the proper terminal in the trailer socket on the vehicle, and the wire you are referring to which must be connected to the vehicle stop light circuit.