I had the same problem on my a/c equipped 79 coupe once. If I remember correctly, there is a resistor for each fan speed under a little cover near the top of the a/c unit. I replaced the resistor and it worked normally. You could probably test the resistance of them just to be sure. If I'm wrong, the resistors may just be used on the lower 2 speeds of the fans, to dissipate the extra current, then on "high", the whole thing is bypassed. But, again, I replaced mine, and all was good.