Well, if you have bucket seats or seats where the back can be folded forward, you can use the seat back as a switch:
Run a conductor from the battery and up one side of the passenger seat bottom, leaving enough conductor exposed on the back edge to make an electrical contact. Do the same thing from the light to the other side of the passenger seat. Put a conductor across the bottom of the movable portion so that it touches the contacts. Seat folded forward for lights off, folded upright for lights on.
You could also rig a rotary switch to be turned by the steering wheel.
Phil Schenfeld - with a minor idea or two