Heater Issues
 

When I crank the temperature knob all the way to the hottest setting (27C) I don't get any warm air from any vent. With the motor at operating temperature, the blower operates normally, and even blows hot air out the wheel wells like 964's do.

Does anybody know how to diagnose 964 HVAC issues? I fear that my recent stereo installation could have messed up a wire somewhere. Specifically, I would like to know how to test the heater rheostat, but would settle for knowing where the control box is located so I can trace the wires to it.

John,
Your 'Control Box' is intergated with your heater panel and testing it usually requires the use of a Bosch Hammer. Beyond that an intimate knowlege of the system will be required to diagnose.
If the problem started at the same time as your stereo instalation that's not a bad place to start looking.
Checking some basics before you worry too much about rheostats is a good idea.
Have a look at your fuses and also pipe connections to and from your mixing flaps.

Mark

Is it cold just one one side? If so you have a bad temperature contol motor.