[JRTJH]

I would first use a 15 amp (blue) fuse and jumper between pins 3 and 4 on the trailer umbilical connector. Those pins (3-4) are the ones immediately adjacent to the "key" or "indexing bump" on the connector. Pin 3 is the tail/running light connection and pin 4 is the 12 VDC power lead (from the trailer battery) so when you jumper those two pins, you WILL illuminate the running/tail lights and "IF" the trailer wiring is operational, those lights will work. If the trailer running/tail lights do not work, or if you blow the fuse, you'll know there's a problem with the trailer.

NOTE: Be sure that the trailer "battery disconnect switch" is in the ON position and there is a charged battery attached to the trailer for the above procedure.