So a couple weeks ago, in my '70 non-A/C. I got about a mile from the house, I start smelling something burning of a non-electrical nature. Within 20 seconds, the cab was FULL of smoke and I had to pull over and shut it down! No burned wires, no HOT wires, nothing smoldering, literally NO sign of catastrophe! I limped it home and parked it. Same thing happened a week later. So now I've discovered that the smoke is coming out of the lower heater vent as the truck warms up. The blower motor is not on. I've actually put my hand on it and it's not even warm. The wires aren't hot either.
Now, I'm sure there are 300 leaves, etc in the heater box, but in cold weather when the truck isn't even up to temp, there's no WAY that stuff could start cooking... Every truck of this era would have burned up by now if that was the case.
So who'd like to make a prediction of what I find when I tear this thing apart??