Trailer lights

If some of its running lights work and other don't, it's the trailer.
If none work then it could be the truck. Need to get out your multi meter to know for sure!

Check for power coming to the lights first but probably a poor ground will be the issue.

My guess is a bad ground also. A clue is if the lights that do work are brighter than normal.

take none working lights apart clean things up the ground is most likely the problem.

ESDA wrote:
The break lights and turn signals all seem to work.

Run a temporary ground to the light fixture. If the light works, it is the ground. If it still does not work, you have a break in the hot wire.

If this test shows it is not the ground, start at the last working marker light. Check behind it to see if the hot wire continues on and the connection is good. Go to the first non-functioning marker light and do the same. The problem is between those two