[Jdub]

Your setup may differ slightly from my '78 but here goes.. First, do perform a Search on Flapper Box to begin understanding how the heat works.

The system is a bit lame - engine heat is blown through a pair of large hoses you will see when you look at your engine. You decide to allow all part of the heat in using the pair of levers between your seats. Full up is full on, full down is full off. If all is working as it should, the moment you lift either lever, the heater blower motor is grounded and begins to blow through the hoses, augmenting what is arguably an MG-grade system.

Problems arise when the wires to the flapper boxes that allow/prevent heat to the cabin break or become sticky, and when the actual rounded cap plates on the flapper boxes decide to rust to their pivot pin. Now, you have a permanant heat on or off depending on the state of the cap plate. Next, the blower motor often will not work because the switch under the levers, which is plastic, degrades over time and finally packs it in. Finally, that same blower motor in the engine bay (look upper left) will often pack it in or worse, run and melt the wiring at worst.

You are getting alternate heat - sounds like one flapper box is not dumping the heat but instead is driving it to the cabin. Compounding this is the fact it *appears* there the heat is blowing in - sounds a bit like the blower motor is on all the time, but noshing is simple - it could easily just be how fast your engine is running. You see, the heater blower motor augments the flow of air, but frankly many find that, at higher revs, they are getting plenty of heat since the fan in the engine forces hot air through depending on the speed of the engine. They decide the blower motor is not needed and pull it totally. This is referred to as a "Heater Backdate" for our cars, since it makes them work much as the older Pcars (and your old VW).

So get the car on a lift in a GOOD PORSCHE SHOP FOR GOD'S SAKE and have a look.