AC Heat switch controls

[el_gureo]

I have a 1972 truck. It’s was a non-AC truck but the previous owner added AC to the truck. I’m not sure how this is normally done but they used the Air/Fan control for the AC. So the AC has two speeds. That’s ok, but what valve does the Air Fan lever control?

I ask because when I get the truck I tested the AC and heater and they worked. So the summer while going down the highway with the AC on heat was coming out of the floor. I assumed that as I moved the control lever over its opening a “heater” valve. So I disconnected the cable to the Air Fan control lever. Great no more heat blowing, but now the heater doesn’t even seem to work at all.

I get that the “heater” fan switch is now the AC fan switch but how do I fix this? How can I make them both work?

Any help would be appreciated.