You can ground the wire, but I dont see any input for that on the temp. control unit. You might want to check your temp dial. Maybe the PO fiddled with it and put it back the wrong way. Also the copper strips could be worn out on the dial or it may need a little bit of cleaning. So it wouldnt give out heat even if the dial is set at max? What about at DEF?

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'85 300D Cal 280,000 miles
'14 GLK 350 60000 miles