The seat belt interlock relay wouldn't interfere with starting after 1975; in normal operation. During 1974 one had to buckle up to get the car started....that went over like a lead balloon and was changed the next year on almost all cars. Many of those systems had the seat sensor disconnected, which worked fine for decades. I still run across a 74 every year or two that the control unit fails after waiting for someone to sit in that seat. Most current techs never knew the possibility for a restrained start existed.
I had a car with an intermittant "no start". I drove and started the car for days (probably replaced the N/S switch) and could never get it to act up. Finally one day a customer called needing a ride and I went to pick him up with the car. When we were inside the car it wouldn't start. Luckily I put two and two together and had him get out of the car. It started right up. Come to find out that the car had the seat upholstered and they had reconnected the long ago disconnected passenger seat sensor. When ever someone sat in the passenger seat (and didn't buckle up) the car wouldn't start; just as it was supposed to do....
On a 78 model the seat belt interlock relay still probably had the start circuit running through it (I would have to look at a diagram). If you are in the CD go to the chassis section in group 54 or 82 and find seat belt interlock system. There were many versions.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician
[This message has been edited by stevebfl (edited 06-17-2000).]