It's most likely a poor connection from your vehicle to your
trailer. First check to see if your trailer connection is corroded
if you can scrape the metal a bit it may give you a connection.
If you buy a 12 volt tester ($4.00) with the lights on attach the
alligator clip to the ground which is the white wire or the exposed
metal part on your vehicle connection put the sharp tester part to the
other "holes" or connections on the vehicle. If you have power the
tester will light so turn on your running lights and caution lights.
If this is working the problem is not your vehicle. good luck