1984 300SD Blower failure odyssey

[914four]

I am at wits end. Last summer (2010), we bought a 1984 300SD because the 240D's trunk wasn't large enough to accommodate my mother-in-law's walker and my son's baby stroller together, so any family outing required two cars or that one member of the family be excluded. When we picked up the beautifully maintained SD in August 2010, the underside was cleaner than many late model cars I've seen and without a spot of rust save for the exhaust system. The paint had been touched up with a poorly matched silver on the hood and behind the two front wheels, but other than that the car was in top shape. Everything was fine until late September when the climate control blower motor stopped working. I replaced the fuse and it worked for a few days and stopped again. We took the car to a local mechanic who had done basic maintenance on the 240D (such as the brake disks and a few oil changes I didn't have time to do myself) and seemed to have a handle on MB. They replaced the blower motor, and the blower motor control regulator, and ran a separate line to the fuse box with a 25 amp fuse. They also told us the A/C was finished, and would cost several grand to repair.
Living in Canada, and the car being equipped with a sunroof, that's not such a big deal, but heat is, and so is defrost. A couple of days later, the blower stopped working, and upon closer inspection I found that the fuse had not only blown, it had melted the fuse carrier. A quick search on the internet revealed that they had probably bypassed the resistor, which led me to question their work. A closer inspection showed some poor electrical practices, wire junctions loosely wrapped in electrical tape that were just a recipe for a fire. Also, the electric antenna stopped working, but they claimed it wasn't working when we brought it in.
After that, we brought the car to another mechanic (over an hour drive away) with more experience in MB, and they undid everything the first place did, then replaced the in dash control unit with a used unit. The blower worked for a few days, but then melted plastic fuse #10. Replaced it with a ceramic fuse after cleaning the contacts with a brush and worked for a few days, then the blower failed entirely without blowing the fuse. Brought it back to the mechanic, and they replaced the speed resistor pack. It worked fine for a few days then failed again. Brought it back and they held on to it for a week, and when we picked it up it was fixed, or so we thought. They had repaired it, but it worked intermittently for a period of a few months (worked when temps were moderate but not in extremes of hot/cold) but then there was nothing. Also, around this time, the dash was randomly losing power, and the running lights were all failing at once, but cleaning the fuse box with a brass brush and some contact cleaner seemed to resolve these issues. The fuse box looks spotless by the way, it is so clean it looks better than the one in my 2009 Jetta.
Reading the forums, I discovered that the ignition switch could be the trouble, so brought the car back and they replaced it. The blower worked for less than a day and stopped. Also, the old switch doesn't seem bad, but since I asked them to replace it I can't complain.
In the summer months, I can do some work myself, so I have tested the motor, and even bought a second as a spare, both work when connected directly for power. I have tested the sensor at the top of the windscreen by removing it and blowing on it while measuring the resistance (I also have a used spare and they give similar values). I have disassembled the in dash control unit, and discovering that one of the pushbutton switches was defective I replaced it with one from the other, testing all operations and refreshing all doubtful solder joints (all with a temperature controlled soldering station @ 700 degrees). I have loosened, cleaned and re-tightened the several grounding points behind the dash. We have not replaced the monovalve, but the MB mechanic didn't think that was the issue. Can the monovalve prevent the blower from working? I am tempted to have them replace it since it seems to be the only thing that we haven't replaced!

Here are the symptoms. A/C/heater blower is inoperable, no matter what setting I select on the fan speed control or on the system selector. We get heat flowing through the vents when at speed and the air pressure is pushing it through so the basic system seems to be working, just no blower. If I twist fuse #11, then press a button, there is a “puff” from the blower, as if it is spinning a few revolutions and stopping. Sometimes it will do this at startup (that's why we thought the ignition switch was bad). There is currently no voltage at the 3 wire plug at the fan motor in any settings. There is no external fuse, the one that on later cars would be on the drivers side just in front of the master cylinder it's in its own black rectangular box (I've seen pictures, and looked for it.) I have cleaned the entire fusebox with a brass brush and tested each fuse, as well as having switched the relays around. The car is incredibly clean, even after a winter of driving in Canada there is still no rust in the engine bay aside from exhaust, springs and pulleys. We had the car rust proofed with Krown when we first got it.

Items replaced include the blower motor, blower motor control regulator, the dashboard control unit, the temperature sensor, the ignition switch and the speed resistor, (the one that sits on the drivers side just under the master brake cylinder). At this point we have spent more on the climate control that what we originally paid for the car, and my wife wants to get rid of it.

Any thoughts or advice would be greatly welcomed.