Ok,,,sound like more of a fuse problem then a wiring one,,,if from what you have said you already changed a blown fuse which means that wiring was good enough to override the fuse first,,,
There are some fuses in the underhood fuse box that refer to the trailer, backup trailer lamp at fuse 4, trailer at fuse 32, trailer turn signals at 50 & 51. There should be a map to the fuses and relays inside the box cover.
check all fuses,,if you are not sure if they are blown you can check both sides with a testlight hooked to ground when in the vehicle or with an ohmmeter across it when removed from the vehicle,,
hope this helps,,,if you find that a fuse keeps blowing when you put it in,,,remove the suspected bulb from socket and see if it still blowes,,,if it does then you will have to eliminate the socket from the circuit and see if it is ok after that,,,you can do this by cutting the power wire to the socket in a place that is easily repaired if the socket is determined to be good,
hope this helps
Oh ya don't forget that there is a fuse and relay center under the driver side rear seat (at least there is on mine) check there too