I just bought this truck and tried to hook my.

I just bought this truck and tried to hook my boat trailer to it. Bought the adapter that has the built-in test lights for the factory 7 plug. I connected the trailer and all the lights work except the right turn signal. Took back the adapter and got a new one and it still didn't work on the trailer or adapter. Checked the wiring behind the factory plug and the wire adapter to make sure it wasn't loose or looked fried anywhere. No luck. I am unsure what fuse, relay, wire connector or module I should check next. This is a 2002 Ford Expedition 4X4 XLT 5.4 V8 Triton truck. Please help! I am trying to get ready for a Catfish Tounament and need my lights working properly.

*Note: All lights in truck work properly even when trailer is connected. Trailer lights are new and have even replaced just to be sure.