trailer wiring question

i bought a trailer from a friend. he just rewired it. i hooked it up to my jeep and all of the lights work great, except when i hit the brakes. when i brake the passenger trailer light goes off, and the drivers side gets dimmer. any ideas?

 

Check for good ground wire connection. A lot of trailer lights ground through the mounting bolt. paint & rust prevent a good connection.

 

If it's a 4 prong flat plug, the white wire is ground. Even though it will ground thru the trailer ball, it still should be grounded thru the plug.