I had a similar problem.

What I found was that where the heater control valves operate, there is a small copper wiring around them that engages/disengages the magnet to move the valves up/down. This will effectively control hot/cold air.

I completely disassembled the units and found that coolant had leaked out of the small rubber gasket and into the copper wiring. One of my valves was constantly shorting out and was jammed ON. The result of this was hot air all the time on one side, regardless of if aircon on/circulation on/off / whatever.

I tried to clean the entire unit and get a good circuit running, but because the circuits on both sides need to be complete for the valves to function, I just threw it away and bought a replacement for £20. Works now.

If you want to diagnose, dismantle the valves to the point where you can see the copper wiring and use a volt meter to check the AC going through; it should adjust in accordance with the heat setting in the cabin.