Starting to lose heat in my 300SD

[otto huber]

I got the AC working in my car about a month ago by replacing the monovalve with one I pulled from a junkyard. At the time I was only getting hot air, and after the repair I was getting every temperature that I selected on the wheel and air was blowing through all the vents with the exception of the center vents from time to time. After reading a lot about this HVAC system, I realized that I'd be lucky if this system would be perfect for more than a month.
About two weeks ago I wasn't getting heat on the economy setting, but the blower was blowing fresh, cold air. I pressed defrost and got heat, then went back to economy and got heat again. This scenario happened a couple of times until today when I went to hit defrost I didn't get any heat. After I let the car sit for a few hours, I went out again and had heat in the economy setting. I stopped to fill up with diesel, and when I started the car I heard what sounded like rubber stretching in the center console. All I got was fresh cold air on the way home. I tried the defrost trick, and only got heat on defrost, but not economy.
Is there a flap that's not opening to allow the hot air in? If this sounds like a CCU issue, is there a way to test the ones that are in the junkyard with a multimeter? Thanks