Can't be too complicated bro- but if all else fails, you can always sell that puppy to me I'd love to buy some pre-1980 car/pick up for a project. you have an original radio i take it? if you take a picture of the back, i'll see if I can help you with that.
I have an 83 datsun/nissan truck. It had an AM radio with only 1 speaker. The wiring was a bit weird. It had power, ground, light (power from headlight switch) and speaker wires. When I was replacing the radio I had to run all new speaker wires. I believe it used a common - wire or maybe even the vehicle ground. It is tough though. Good luck on the diagram.