Rear right turn signal/brake light not working
I am having an issue with my right rear lights. When hooked up to the trailer, the rear lights come on along with every other light including the right rear. However, the turn signal and brake light don't come on on the right. I tried with 2 different vehicles, which I could assume its not on the vehicle side. I switched the working left hand bulb with the right and still the same result. I don't know where to go from here. The light mount is sealed to the trailer, so I am hesitant to break the seal as to avoid leaks.
Can anyone point me in the right direction on how to find this issue and resolve it?
I appreciate your input as always.
Rebuilt 2001 Trail-lite Bantam B23SB
2021 FORD F150 XLT