turn signal worrks when the lights are off

Do all the lights work on the left side, when turned on?
Fuse?

 

Man o man, I just chased this exact same gremlin in my Western Star. It ended up being the LED fender mounted turn signal. The marker light on the side of it"s wire was crimped between the body of the light and light itself. It came like that from the factory, but took the vibration of the truck a few months to start shorting out. I also had an issue on the other side doing this where some wires had rubbed and started shorting out. Turn signals would work in both instances without lights but not with. 4 ways would work half the time with lights. I see some trouble shooting in your future. If you always pull the same trailer, pull the pigtail to eliminate the trailer being part of the problem.

 

You have a ground problem.

 

Do you have LED bulbs, by chance?

 

You left out a major part. Does it malfunction ONLY with a trailer hooked up, or does it do it also bobtail style?

 

Its always the easy stuff, ain't it .....glad you got it fixed....