This diagram may help:
Trailer lights ARE evil. The very first thing to do is to make sure there's a good ground. Use a 12V circuit tester to make sure there's power at the plug on the back of your vehicle.
You should check trailer light fuses if there's no power at the back.
I had a problem a couple of years ago. Some light weren't working, I was using a round 7/flat 4 adapter. It turned out the problem was in the adapter. Go figure.