The circuit design is done and I'm halfway through the microcontroller code (all 20 lines of it). In case anyone thinks that a microcontroller is overkill for this, instead think of it as a 99 cent timer chip. Seems less intimidating somehow.

It fits in the same box as the wiper module. The parts are less expensive than in the wiper module so this guy will be $22 delivered.

This is one gadget I can test on my own truck. If it doesn't go stupid in a week I'll consider it done. I'll build some on vector board for a select few to try and then I'll get a PC board made.