I've diagnosed my heating problems to the radiator fan, it won't turn on when the engine is hot. When I turn the AC on (my ac is taken out) the fan turns on, and the engine drastically cools down, so for now I don't need to make my own switch. But I don't know where the relay sensor is, I'm guessing the sensor broke. I've got a few pictures showing some wires that I found that I think may be the wire. Engine isn't stock, and I don't think the fan is either, but both are from the 80's.

 

Best thing you can do (in my opinion) is get an aftermarket fan controller - it's one of the best purchases I ever made. I have a Flex-a-lite controller, fully adjustable temp setting, probes the radiator for temp input reading, has provisions for a manual on and/or a manual off switch that overrides the temp sensor - was about $75 I think?

The factory fan switches never worked for me, tried expensive aftermarket, GM, el cheapo AZ junk - never got one to work more than 2 mionths. Got tired of getting hosed in coolant everytime I had to replace one, and went with that controller 4 years ago, haven't had to look back since!