check the "toad wiring" thread for info on my struggles with this.
I have a Saturn that was wired by the po. My Airstream had a flat plug that was shot so I replaced it with the type found on the Saturn, a round 6 pole.
The thing would not work. Tried all kinds of connections, until I found out that the ground wire was not connected right. When I grounded the motorhome AND the car, the lights worked. Although- one side is still out, and I believe it needs a new wire run. I used my test light to try each plug hole, and also applied voltage to the wires to see which light would come on. Same for the motorhome, test light into the holes, watched to see if the proper trigger lighted the light. (my son held it while I stepped on the brake,etc.)
The wires on mine run through the firewall, to the trunk where they are tapped into the light wires. I would also consider the portable lights that you plug in and set on the roof ( it is metal), as they would be a whole lot easier to deal with.
I am still considering that option.