When the car was fitted with factory AC, a micro switch was fitted in behind the fascia. Pushing the Fresh air/Recirc Lever to the right would turn off or on, the micro switch to activate the AC - provided you had the fan on at least low speed.
The cable may have seized up (i have never seen this happen before but there is always a first) or maybe someone has "tweaked" the leverl along the way. The only way to find out quickly, is to check your fan, like I did last night to take the pic, and check if the cable is connected to the fresh air flap lever I pointed out to you.
Other wise, its disassemble the controller section of the dash.
If you're mechanically minded should take you about 15 mins to get access to the heater control unit.
Remove the following: ( i may have forgotten a couple of screws but you should get the idea)
- two Philips screws up above the heater controls - they screw into the under side of the dash.
- all knobs from heater control panel ( I use pliers) - there is a tiny Philips head that holds the Recirc knob on.
- Radio knobs and the 12mm hex nuts and washers on the tuner and volume controllers.
- the ashtray
- the little coin holder below the radio ( Disregard if your 626 has a cassette player)
- carefully lever the top section of the switch control fascia down from below the dash, to allow it to pop forward. BE GENTLE - or Baby Jesus will cry when you crack the plastic!!!!
- carefully unclip and pull forward the whole fascia so that it gives you access to the heater control unit. To completely remove, disconnect the lighter wiring and defrost switch wiring.
The heater control unit is held in by a couple of screws and the actual cables ( clips anchor them) - 1 to the fresh air/recirc flap on the blower, 1 to the heater tap RH side of heater box close to the trans tunnel and 1 to the air mixer about half way up on the RH side of the heater box.
See how you go!!!