I just got a 05 SCrew with the 7 pin connector on the trailer hitch. Last night I noticed I had brake lights and turn signals coming through the tailliights on the trailer but I didn't have any of the amber and red marker lights on. My headlights were on so they should have lit. Any reason this would be?

 

Start with checking fuses....if they are good move on to the truck wiring.

Start with the truck and a test light at the trailer plug. Turn on your park/running lights as well as your hazard flashers. This way you won't need to leave your key in the on position. The running lights will be hot and the right and left turn/brake will be flashing.

It sounds like the turn signal/brake lights are working, so concentrate on the running/park lights.
For a 7 round:
11 o'clock--running/park lights
3 o'clock--right turn/brake lights
7 o'clock--gound
9 o'clock--left turn/brake lights

If everything is working well it will be time to look at the trailer.
When testing make sure to use the ground in the trailer plug connection rather than the bumper or another piece of the truck. If the ground in not functioning in the trailer plug you will miss it by grounding out the test light on the bumper.

 

if the truck was equipped, make sure the automatic lights are not on. i found that to be my problem when i was towing my 4 wheeler

 

Fuses, fuses, fuses. All but two of my trailer lighting problems have been blown fuses. One was bad wiring (old owner), and one was a bad bulb.

 

Heed this wise man's advise. Check these 1st!!!!!!!

 

Had the same issue. It was indeed blown fuses, however the cause was a rubbing wire on my travel trailer that I had repaired under warranty.

They would work for a while after replacing the fuses and then would eventually blow them again.

Did the same on my Sequoia at the time so I was certain there was a trailer problem and sure enough it was.

 

Make sure the trailer park lamp relay is installed in the fuse panel under the hood. It's one of the smaller "half" relays.