This is the schematic for the A/C in 82 SC, It seems that the third position is effecting the 12volts being supplied through fuse 20 (25 amps.) Fuse 20 supplies the power for the A/C clutch, front condenser cooling fan and the three position evaporator fan. If you understand how to use a multimeter I would recommend doing a continuity test without power to check for a short to ground in the A/C fan switch and evaporator fan resistor unit.
