1975 Dodge Dart won’t start

I recently bought a 1975 dodge dart Hang Ten and installed a new ignition starter switch. The guy I bought the car off of swore the car had issues starting and that it had to be the switch. I replaced it and it will occasionally turn over but not fire. Well I tried wiring, rewiring, and rewiring and the same issue. While I was working on the car I accidently turned on the heater and boom it starts every single time. I have no idea why and any help would be appreciated.

Are you saying that The engine will only start if you have the heater fan turned on???

Yosemite

Yes. It will only start if the heater switch is turned on.

Well, that one has me stumped. The only thing I can suggest is that you check all grounds.

I would recommend that you buy a spare ballast resistor and keep it in the glove box. They are the usual cause of a crank but no start for these cars. If you check for spark and you get one spark and no more while cranking, it is the ballast resistor. They are cheap.

will do thank you