I have an 02 Tahoe with the factory installed 7-lead trailer wiring harness and I am using a trailer 7 pin to 4 way flat adapter to connect to my boat trailer.  I just got the Tahoe so I have never tested this wiring before.  Initially when I tested the lights everything worked except the right trailer blinker.  I discovered that a fuse under the Tahoe hood for the right blinker was blown.  Once I replaced the fuse the trailer lights, brake lights, and blinker lights work just fine when my headlights are off.  As soon as I turn on the headlights (or they turn on once I take off the e-brake) the blinkers go crazy.  When I turn on the right blinker the Tahoe blinker works just fine but the trailer blinker go back and forth like a police car (right, left, right, left, etc…)  and the same happens with the left blinker.

 

Any ideas what this could be?

the wires are crossed up is a good possibility