First, let me say that I have limited experience with the climate control system on the SDL. Mine also has "issues" such as no floor heat and a compressor that likes to run in economy mode.

However, in your situation, it appears that the control unit is not getting the proper signal from the vehicle and does not realize that it needs to turn itself down when the vehicle reaches the desired temperature.

This could be caused by a faulty air sampler motor in the right doorjamb. It's a tiny motor that draws air from the intake near the sunroof switch and provides the input to the control unit. Take a piece of tissue paper and see if the tissue paper will remain on the intake grill when the vehicle is running. This will tell you that the sampler motor is drawing air. If it does not stay on the grill, then proceed with finding and replacing the sampler motor.

My sampler motor has been screaming of late and I'm sure it's going to go out any day now.

One other possibility when you have maximum heat all the time is a defective monovalve. This is the valve underneath the hood that regulates the flow of coolant into the heater core. If the diaphragm fails (common), it sends coolant through the core all the time and you will quickly roast inside the cabin. It's not difficult to take the monovalve apart and repair it.

These are two possibilities in a multitude of possibilities on this system.