I'll let others who are more knowledgable play clean up here.
I "know" it's for the seat belt system and I believe the system does the following:
Disables the ignition if someone is sitting in that seat and the belt isn't fastened (check for wires shrinkwrapped down the seatbelt receptical)
Disables the ignition if Slits is driving (naked)

Sparks the little light on the dash if you're not buckled up.