I'm assuming a 74? First off if you want to bypass the system, just disconnect the plugs that goes to the seats, that will disable the weight sensors and the car will always think the seat are empty. There is also a bypass under the hood where you can stick a pencil to start the car for service. The manual details it.
When the car doesn't start...does the Fasten Seat Belts light come on?
Did you connect the wires both to the seats and to the female end of the belts (mounted on the hump)?
For troubleshooting mine I left the driver's side unplugged and the passenger side plugged in. I would just take a heavy tool box and set it on the passenger seat. It was pretty easy to just lift the box or fasten/unfasten the belt when I wanted to test the various conditions.