Welcome to the world of Series 1 heating and cooling. The A/C is one of the simplest you will find but can be a pain. RustfreeMike is correct, The control on the left is an electro mechanical switch that incorporates a capillary tube for temperature control. The switch goes through two A/C relays mounted just under the dashpad behind the Tach. They turn the compressor on and off as well as control the fan speeds in conjunction with the fan switch on the panel. When you turn on the A/C the fans go on even if the panel fan switch is off. My car is wired so the fans will run with the A/C knob on, even if the ignition is off. Dead battery alert. I am not sure if this is normal. I have never been able nor do I want to try to figure it out.
When working behind the panel, be extra careful with the other controls. The rest of the system, the heating, is all vacuum controlled and the rotary control to the right of the radio and the fresh air vent lever above the radio are very delicate. It is very easy to break the plastic vacuum nipples off. They can be repaired if you are good with epoxy and it is almost impossible to find replacements. If you do they are very expensive. The vacuum hoses are a rats nest back there. Be careful pulling on them. If you can, replace them with new hoses. Most people, myself included bypass the thermal control that is attached to the heater box.
Good luck. I don’t envy you. It is my least favorite part of the car to work on.