cjoe,
I think you must remember, that anything that conducts electricity could be thought of as a wire. Technically, the entire frame of your truck is a big wire.
When power goes into a lamp, and can't find the conventional ground, it will head back through the other filament, down the wire, into the lamp on the other side, and out through the lamp base to ground.
As it does this, that opposite light will illuminate, slightly, as current passes through it , using its properly grounded base to return to zero potential.
I think this is how it works. I was enlightened by Baldy and Cletis just last week, so I might have it wrong. If you draw a little cartoon of the lights and wires, you can trace a path that makes it more clear how this bizarre action could take place.
Stuart
