No heat or ac

I have a 2009 pt, the heater puts out very little heat, the ac worked fine until this summer when it gradually got less effective.

The blower works as it should, the duct control works as it should. I have replaced the thermostat. The car has no overheating issues always runs in normal range. I’ve tried recharging the ac but it won’t take the refrigerant. I suspect the temp control knob on the dash but I have no proof and I can’t seem to find a new replacement. How can I identify the problem for sure?