I'm in the same boat
 

I just joined the forum, I've been going crazy trying to figure out my heater issues and it sounds like we are having very similar symptoms. I just got the car a few weeks ago, heat has been inconsistent until recently when it stopped all together. I replaced the torn monovalve and checked operation while it was out, seems to work just fine. Occassionally i may get a little bit of heat to come out of it, but then it goes cold and shows no signs of being warm. However, the past few days i cant even get it to blow heat at all. Pulled the glove box, vacuum flaps appear to be working.

I felt the heater hoses after the engine was hot and ACC was on full heat. Hoses on engine side were hot, hoses coming from heater core were ice cold. Next i will check to see if the heater core is flowing at all, maybe bypass the monovalve just to be sure. Anyways, please keep posting if you figure out your problem.

Installing a brand new Bosch monovalve (complete assembly) from $Mercedes Benz$ fixed it. Part number is 000-830-29-84. Don't waste the time/money with the ones off ebay.

Thanks for posting this. As doubtful as I was that it could still be the monovalve I decided to bypass it this morning anyways and what do you know, HEAT! finally! It looks like I'll be in the market for a new monovalve assembly.

hopefully you can find a good used one. I have three sitting on bench that don't work. MB retail is around $500.