Oh uh. Do you have the dreaded seat belt interlock on your car, that lovely system around for a couple years that prevented starting the car unless there was a driver in the driver's seat and the seatbelt was latched?
Seems that was 74-75, somewhere along in there.
What you need to do is see where the voltage is in the starting system -- from key switch to whereever it goes. I don't suppose the thought police will lock you up if you defeat the system by wiring direct from starter key switch to neutral safety switch!
Do check that you get continuity through the switch. Burned or corroded contacts aren't the only thing that will keep a switch from working...
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!