PUP lights and electric brakes not working

As mentioned previously, clean the contacts on both the tow vehicle and the trailer cable. If that doesn't fix it, then the wire terminations inside trailer's plug. There probably is a small set screw on one side of the housing, remove it and slide the outer housing back along the cable toward the trailer. Check to see that the wire terminations are all clean and tight. A flat blade screwdriver should be all you need. Replace the outer housing and replace the set screw.

I had an issue with faulty tail lights that was caused by a lose ground wire connection inside the plug. 5 minutes is all it took to solve it.