Well, I managed to score a new Bosch DEF heater for $382 plus free returns (if that turns out to not be the problem and it is wiring or something else). Gonna drop the DEF tank this weekend and do the job, if the wife lets the thing stay parked long enough for me to work on it....... Pretty sure the heater is toast as it is original to the car, and the same code (P20BD) keeps popping up whenever it is cleared.

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2012 Mercedes ML350 Bluetec 102K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 238K (mine)
1998 Volvo S70 T5 Turbo 211K (kids)
1994 Ford F150 4WD 246K (firewood hauler)
1983 Mercedes 300D 384K (diesel commuter)