Looking at the wiring diagram, there are a few small items that I'm not familiar with (e.g. "Magnetic Valve"), but it seems that almost everything about the front and rear climate control systems are separate. If you're having problems with both, the only thing I can possibly think of (other than the control unit...) is perhaps a problem with the CanBus. Roverguy7 seems to be pretty knowledgeable in that area. I don't know if he's still on this board, but you may want to shoot him a PM or something and try to pick his brain.

 

Have you had the vehicle scanned? The HVAC Control Module stores faults for the blend motors, and even system pressure if too low. I've replaced a few blend motors and seen blend door bidding as well.

A quick google search gave this thread.

DISCO3.CO.UK - View topic - Heater Blend Motor

 

Peter, thanks for the post. My AC is actually low. I note a leak out of the back plate on the AC compressor, lots of oil. I haven't had time to pull the AC Compressor yet, it looks like a pain. Is there any way to simply remove the back plate, replace the seal and reinstall the back plate while the ac compressor is on the truck? The leak was detected about the same time as the vents/blend doors started going haywire. The passenger side has hot and cold. the driver's side has hot and outside temp only. All face vents do not work. Everything was working great and then bam, the face vents didn't work and the drivers ac stopped. By the way, the windshield vents work and the footwell vents work on both sides, the face vents don't work on any of the four vents, drivers side, two middle or passenger side. thanks phil

 

I've been trying source the compressor seals without any luck. Denso sells built compressor with the seal. Yes, the compressor is a really pain. It is recommended to replace the receiver/drier with the compressor. The A/C is will work poorly with a leak, even a very smal leak. I've seen a 15 to 20 degrees difference between vent temps due to low freon. Eventually, the A/C will stop blowing cold.

From the sounds of your description, you may have a few things going on with the HVAC. The driver's side blend is actually the easiest to gain access. The lower steering column trims come off and expose the side of the A/C box. The driver's flap motor is right there. Remove it from the box and watch to see if the motor moves. Also, check the flap moves in the box.

As I mentioned early, sometimes the box flaps get stuck and the box needs to be replaced.