I have a 99 Jimmy 4WD. I have a trailer hooked up right now and have a strange problem. When I turn the headlights on, the trailer lights come on, but the brake lights and signals don't work. I have to haul a bike this afternoon, anyone know what would be causing this?

 

Are you using a 4pin plug or a 6/7 pin plug? I am going to assume that you have the 4 pin plug as you didn't mention anything about reverse lights or the other features present in a 6/7 pin plug.

Here are a few illustrations of the wiring so you can probe at the connector to see if you have what you need to have.

As you can see, the green wire is your right stop & turn. Yellow is your left stop & turn. Brown is your tail, marker, license lights. And Black is the trailer ground.

For the brakes, the green and yellow wires should have +12V constant to them. For the right/left turn, the green/yellow (respectively) wire should have intermittent +12V corresponding to the blinking of the turn signal. Whenever your marker lights are on, the brown wire should have +12V constant.

Since you have the marker lights, your ground should be ok.

Hope this helps!