Steve,
The relay can be a problem.
My 77 had a 2 or 3 wire harness that laid across the engine.
The harness has (had) a connector in it, the connector would over heat and melt, causing a problem, I chopped the connector out, butt connected those wires together!!!
Another issue can be corrosion on wire connectors, especially on the resistor.I know there are some differences on the A/C setups between a 75 and 77, but
most of it is similar...I replaced the dash switch, once in my car, it fixed that problem, that time....
I'm going to remove (if possible) all the A/C and heater wiring under the hood of my car and give it the once over here, when I get a chance. I'm thinking of soldering some of those wire connectors, so corrosion won't get between them...
__________________
1977 Black Trans Am 180 HP Auto, essentially base model T/A.
I'm the original owner, purchased May 7, 1977.
Shut it off
Shut it off
Buddy, I just shut your Prius down... 
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