Forgot about this thread....Mentioning the seatbelt switch reminded me that on the '74 Trans Am/Firebird, there was a seatbelt switch that went to a relay on the driver's side inner fender. If that thing goes bad, you definitely will not engage the starter.
You can see it on the inner fender on the pic below...right ahead of the master cylinder.
Does your '75 GP have anything like that?
