Well you really need the factory manual with the troubleshooting charts and a HVAC technician. The first thing I’d suspect would be the whole heater and AC switch module in the dash. Rockauto has the whole shebang for around $100. Don’t know if it still uses the HVAC programmer module anymore or if the switch module takes its place. When you go to AC, does the compressor come on so its just a blend door issue? Or does the compressor not work at all? If the compressor doesn’t work there are simple checks to make sure all the electricals are ok such as the low pressure switch, but I suspect you need to replace the whole dash switch.
Doesn’t appear to be a HVAC programmer used so the control unit can be ordered from Rockauto.com or gmpartsclub.com for $133. After years of frustration and $1000, I’d just put a new control box in to try.