Now that the winter is approaching the end, I finally have heat in the Jetta. After fixing the leak from putting the water pump in last weekend (thermostat o-ring wasn’t seated properly), I pulled the dash apart to take a look at the heater core. Once I got it apart, I noticed that the blend doors had nothing left to them except the framing (“swiss cheese” metal) All of the foam insulation was completely gone so there was absolutely nothing keeping the air from mixing. As a hopeful long term fix, I took some cork gasket material and contact cemented it to the flaps and it is a WORLD of difference. Before the car even reached 160* I was sweating inside from all the beautiful heat… lol.
Now the only running issue left is a slight mis / hesitation at low throttle. It happens in any gear and at any RPM so I’m thinking its the TPS.