trying to debug a ground problem.
running a new trailer harness.
hooking just the park lite wires up and grounding the trailer light assemblys the park lites work normal.
hooking in the stop/turn lite wires, the park lights will now operate with no ground! apparently grounding back threw the stop/turn wires?
and the turn signal flashes alternatly from truck to trailer instead of at the same time.
the large plug adapter checks out ok.
the trailer wiring is new.
seems to be factory installed hitch and trailer plug wiring with no signs of melting or butchering by previous owner.
the stop/turn circut is apparently shorted into the park lite circut ?
any suggestion of a test to narrow the problem area?

You need to verify everything at your trucks trailer plug is correct first. A Ohm/voltmeter will do the job just follow the schmatic.
http://www.etrailer.com/faq_wiring.aspx

HAD A SIMALAR PROBLEM BEFORE. THE WIRE IN THE BULB BURNED OFF AND FELL TO THE OTHER WIRE ON ONE SIDE ONLY. SO CHECK THE BULB IN THE TRAILER LIGHTS.

OR IF THEY ADDED ANY OTHER HARNESS INTO THE ORIGINAL AND SPLICED IT, YOU WILL HAVE A PROBLEM THERE.