Look under the dash (just over the throttle pedal) against the heater/ac unit on the drivers side you will see a push rod that is tied to a vacuum canister and controls the internal a/c vents.
That rod has a metal plate fixed on it and when the vacuum is (off, I think) correct the rod is pulled up by the vacuum actuator and the plate comes off the switch and allows the heater fan to come on and turns off the a/c fan. With the plate engaging the switch the a/c fan should run and not the heater fan.

If I didn't explain it well enough let me know and I can take a photo of the switch and location.