https://nastyz28.com/threads/more-a-...speeds.352137/
The thread kinda meanders a bit through the "R&D" I had to do to get to the end result. But I didn't want to retype it all here...
If you don't already have a rotary switch, you'd have to convert to one. I had already converted to a rotary 3-speed switch, so it greatly simplified this upgrade.
But like I said in that thread, it feels like a 21st century A/C system now.
Being in SoCal, I pretty much never run the heater, so I took it off a long time ago, so no heat/cold mixing. A/C is either on or off at full cold.