Bring direct 12V from battery to the heater terminal: does it warm-up ?
Then feed only positive from battery and 'normal' ground: does it still warm-up ?
Finally do the opposite: negative from battery and positive from car: does it warm-up ?
You should by now have insulated the problem ...
As said early 7V is WAY too low, C030A is the common rear grounding point, but I believe several grounding wires are connected to it, so your wiper working ok doesn't mean the heater ground cable is ok !
BRs, Bernie
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