Go get a test plug from TSC, or any parts store, It will plug into the flat pigtail on the end of the truck. There are 4 LEDs that will light up and tell you if it is a problem with the truck wiring or the trailer.
http://ecx.images-amazon.com/images/I/31J3235FZ6L._SL500_AA300_.jpg
I would bet that the issue is in the trailer wiring itself, and not the truck wiring.
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Still rollin with the South Side Ninja