Am I the only DumbF^%$

[piecesparts]

It would be realtively simple to wire something up---- Get a standard relay and set it up to have the power from the headlight switch go through the relay to an alarm, next connect the relay's contact actuating coil to the dome light circuit, so that when you open the door the buzzer will make noise. If you can't find a buzzer in the salvage yard, look for an old car horn and tone it way down.