Quote Originally Posted by coombro View Post
Hit the nail on the head. Grounding big issue. some use the trailer tongue and ball for grounding as some use the white wire.
If you have good connection to turn signals it's probably not a ground problem. You say the side marker and tail lights are not working. The tail lights and the side lights are on the same circuit (powered with the same wire). I would say you have a bad connection with the tail light wire.

Get a cheap 12v tester. Start at the plug connection then the wire behind the plug. If you have power going to the plug but not out of it get a new plug.