You have the recirc mode on? Got to eliminate the easy stuff first.

Electrically is not the heater control valves usual mode of failure. Typically the inside valves get gummed up as the rubber deteriorates over time. Have not seen an electrical failure yet over my, admittedly small, sample size of 5ish. If you are so inclined to open up the old one it is rather easy and would allow inspection. Only "gotcha" is to be careful of the small wire leads.

But you are getting heat in the rear so I'm not sure the valve is your issue. Maybe a diverter flap (the fresh air one for instance) is stuck and not closing?

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