Here's some towing info including the electric diagram for connecting a trailer. Hope it helps troubleshooting the problem.

http://www.whnet.com/4x4/towing.html

I once had all boat trailer lights turn on like a Christmas tree when any single circuit lights should have come on. Turned out to be a corroded ground connection....

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W163.154, M112.942, 722.662, ZF.970.402