69 firebird w/vintage air---found switch in glove box -- assume it controls temperture--what turns the a/c on?

The ac is turned on when you push the bottom temperature control to the far left. The top slider "off" position is the same as having it in the middle "normal" position for a/c or heat. The temp control must be set to "heat" when using the defroster.

The switch in the glove box is for the evaporator.
the fan switch turns on the A/C. Works very well in Florida and the engine runs cool.