Forgive me if this is not the right area for this post. But I recently bought a Roadmaster enclosed 5x8 trailer. When we went to pick up the trailer, we could only get the running lights to work on it. The lights would not brighten when the brakes were applied and the lights would not blink for the blinkers. When we got home with it, we plugged it into our neon and everything would work except the right side blinker and brake. I went and bought a 4-prong connection tester and tested the truck and it checked out to be fine, as did the neon. We also have two new rear light assemblies on the way to see if it was just a bad bulb. My question here is, what is allowing most of the trailer to work on the neon and not the truck? Could it be a bad ground on the truck or is it in the trailer?
Thanks in advance.

 

Could be a bad ground between truck and trailer. Usually grounds trailer to the truck through the hitch, but sometimes not a very good connection for a ground. I always find it best to ensure the trailer side is grounded well from the plug to the frame or tongue area. That way when the trailer is plugged to the truck, you aren't relying on the ground through the hitch. Then ensure truck side for the ground prong or pin has a good ground as well.

Also there are fuses in fuse box near battery under the hood for trailer turn signals, make sure they are good. When I first got my truck (bought slightly used) turn lights not flashing on the trailer, found the fuse and replaced, fixed that issue. So go through and check all the fuses real well.

Additionally, check the ground of each light on The trailer. Some are grounded to the trailer with the mounting bolt, this sometimes gets rusty, and not a good connection. I prefer to run a ground wire from each to the frame with stars washers to bite into the metal to get a good connection.

 

I always run an extra ground wire from truck to trailer. I have the 7 pin connector as part of my towing package so I run the ground from that **** in the connector to a screw that I screwed into the trailer frame. I did this because I keep my ball greased and it doesn't always ground properly.

 

Because you have issues with both vehicles driving the lights properly, I'd go after the harness on the trailer. Look for areas where they compressed/pinched the wire, I had that problem on my 18ft car trailer. Symptom didn't show up for years. Also check that the trailer white wire has a good connection to the frame.

Here is the wire pin out for your 4 prong...
http://www.rverscorner.com/wiring/4pole.html