Running lights 1965 TW

[Dr Dan]

Hey,
Pulled our 65 TW up to Petoskey MI for the maiden voyage last week and every thing went smoothly except that the running lights stopped working at some point. I don't know when, but we got there in the dark and they were out. I whipped out a test light and had blown a fuse in the truck. Upon changing the fuse, the lights worked for a short time, a few minutes, before blowing the fuse again. When changing out the pigtail on the trailer I remembered that there were 2 wires going to the running lights so I disconnected one, wasted another fuse then switched to the other wire and had running lamps on the front of the trailer.
I had all of the upper running lights and license plate lamp "fixed" (replaced) at the factory last month and they worked part of the way home. All were blinking on and off at random so I suspected it was just a loose connection where I hooked it to the truck. I changed trucks (not for that reason) and the lights worked when we left for the trip. I'll check the fuse in the old truck to see if it is blown. I eventually ran a wire from the end of the trailed end of the pigtail to some new running (temp) lamps along with the wire to the front lamps and made it home without incident.
My question is, would there be some way that the installation of the new lamps at the factory could be causing the short. I am assuming that the hot wire going to one of the lamps is grounding out and blowing the fuse. Making taking out each lamp and looking at it my next step, or is the fact that both upper lamps on the rear, the license plate lamp, and both tail lights (running lamp portion) are out tell me that the problem is the common wire feeding all those lamps? Next question. Where can I access that wire before it splits into all of those different lamps?
Thanks,
Dan