One of the first things I did to my Star Chief was try to fix the mess the last owner made of the new carpets he laid in her. While doing this, with my head stuck up under the dash, I found why the heater control worked so easy. It was disconnected, and for good reason. The heater core was plugged.
So, I dug into getting it out.
Remove all the little sheet metal screws from around the heater core box under the dash.
Go out under the hood and remove the heater hoses, then remove the heater core box from out there.
Back inside with the cable unhooked you should be able to pull the entire assembly, core and box, down onto the floor.
The heater core, like a small radiator, can be removed from the box.
Take it inside to the kitchen sink, quietly of course, and run really hot water thru it until all the crud comes out of it. As long as you didnt have any leaks before you removed it, you shouldn't have any now.
Reverse the procedure and put it all back together.
You should also remove the hot water valve and clean that out and oil the lever that opens the valve. There are places to get new valves or have the original rebuilt on the web. If you cant find one and need one let me know. I have seen them in my travels on the web. Ames? Performance Years? somewhere. I cleaned mine and it works great.