rear tail lght glitch (R) works (L) aned trailer tails don't.

I have a 2010 gmc cayon. I have chacked all fuses (good) I have factory tow package also.. I have tail light on pass. side but not on driver side and no tail lights on trailer.is it the plug itself or am I missing something? all others work, blinker, brake,back-up thank you!!