you've had a beatiful plethora of cars, I envy you...
I recently was having the same problem, and thats cos i installed a radio, the groud wire i used, when the lights were off it was a groud, however when the dash and rear runners came on it became 12V.
anyways, im just saying this cos it could be your radio, sounds wrong but try disconnecting your radio from your car, it might be whats causing the short because that wire runs thru it. It's real weird but worth giving a shot if your stuck for ideas.