W126 Heater Blower/Controller

[jcantor]

After suffering through the last several weeks with little heat due to a
nonfunctioning blower, I finally had the time to look into the problem. I've
pulled the blower itself and there doesn't appear to be anything wrong with it. A quick connection to 12 VDC and it spins to life silently and the brushes look
fine. I connected a multimeter to the leads feeding the blower and never got
more than 0.05VDC regardless of the HVAC settings.

I then disconnected the blower controller harness and measured the voltage at that connector - 7.4 VDC at high speed, 1.2 VDC at low speed, and something around 4 volts in the auto position. This leads me to believe that the problem is actually the blower controller (126 820 5310). Since this is a rather pricey item ($400+ even from Rusty or Priceline) I want to be sure of what I'm doing before buying a new one. How can I be sure that the controller is the culprit? Could there still be problems at the HVAC control buttons?

Thanks
Jeff
'87 560SEL