Hmmm...this is before my time but my grandfather remembers it...
Lucky you. I just happen to have a 1974 MB wiring diagram manual (which is really nice and drawn better than later model's manuals). Ironically, I can't use this, I happened to get it with some other manuals that I CAN use.
This car had a "seat belt starter logic relay", which contains the logic:
if (butt in drivers seat and driver's buckle NOT buckled) then open relay
or
if (butt in drivers seat and passenger's buckle NOT buckled) then open relay
The manual doesn't describe the logic entirely-I wonder if the car would start when it was new if no one was in it and you cranked it with an arm through the window?
The relay provides the ground to the AC starter relay, which disconnects the ground to the compressor clutch relay and provides ground to the starter while the car is starting.
The butt switch is normally open and the buckle switch is normally closed.
So, BOTH seats contain both butt sensors and buckle sensors. I wonder if you are sitting in the passenger's seat when you are wiggling the fuse!
If no one had ever mucked with this car, and you are sitting in the passenger seat unbuckled trying to start the car, it might refuse to start even if it is working correctly.
Frankly, since this is no longer a federal requirement, I would bypass it if it was my car. But you might try wiggling the seat belt relay.