I have encountered a problem with my lights that I can not figure out. When I press my brake in only my right trailer light comes on, but when I press my alarm to lock the truck both lights come on. I have changed out the bulb and checked my connection, but I assume the wiring and bulb are good if the lights come on when the alarm is activated. What would cause my brake lights not to work on just one side though?

Any ideas are appreciated, I am stumped.

Thanks,
Wes

Check your grounds and or wiring at the rear lights. Sounds like there is a break in the connecting, re-establish the link, and everything should work.

what tow vehicle ? most have seperate fuse's for turn signals in the tow package . I think what you are seeing on the alarm set is runners.

Ill go out and check the grounds right now, I have checked the wiring at the lights themselves and everything looks undamaged and secure.

It is a 2011 F150 tow vehicle, I have not tried it with another vehicle. I'll look at the fuses as well.

Thank you both, i'll update soon.

Edit- we are in the middle of a rain storm so it'll have to wait for a bit.

Maybe:
if single bulb. brake light filament burned out. tail light filament still good

dang why is it always the left 1 ? if your fuse box is on passengers side kick panel the fuse # is 42 it is in the top left corner behind a relay pull the relay is the best way to get at it ! that relay in the top left corner hides the fuse from view. well unless your 1 ft tall and can get a better view at the panel than a full grown boy, LOL

Brake lights run through seperate wires, same wires as left and right turn signals, so one can easily work when the other doesn't. Alarm is wired to running lights, not brake lights.

So if fuse is good and bulb and bulb socket are good then the issue is either a bad connection at the plug for the left turn signal wire or a bad/broken/shorted left turn signal wire.

I swapped the left and right fuses and still had the same problem.

Also resecured the ground wire with no change.

I am thinking you are right with the bad connection, I have narrowed it down to the single wire that is not working but as far as I can trace it I don't see anything that looks damaged.

It is still under warranty so I guess I'll have to take it in. Unfortunately that comes with a very long wait time right now they tell me.

Thank you for all the suggestions!

Make sure the ground wire on the trucks harness is making a good connection to the frame of the vehicle. The symptoms you describe is a classic bad ground. Also make sure the white wire on the trailer harness has a good connection to the frame of the boat trailer. If you want to ensure you have good grounds on both the truck and trailer take a drill and a wire wheel and grind a nickle to quarter size spot on the frame of the truck to bare metal then connect the ground wire from the trucks harness to that spot. After you connect it you can take some spray paint or undercoat and paint it. Do the same on the trailer and your problem should go away.

I think you could take a set of jumper cables and hook to truck and trailer to see if it is a ground problem?quick and easy !

That would be a good way to check the ground, I really don't think that is it since all the other lights work fine though. Sure doesn't hurt to try it and either it fixes it or rules it out.

Run a separate ground wire from the mounting bolt on the light that is working to the mounting bolt of the light that is not working. See if this fixes it.