I believe you are correct on the assembly of the unit.
How to tell if they're working, well I'm not sure. I do know that on the 560SEL, if I spritz a little water on the mirror and start the car the centers of the mirrors will be dry in a matter of minutes. The outer edges will retain some water drops. I can only detect the slightest bit of warmth on cold mornings (when I touch the mirror after driving to the office).
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.