Finally got the chance to troubleshoot this. I pulled the PBCU and saw that it was really messed up, some wires seemed burnt. So automatically I felt that it must have been defective. I purchased a used one and now it works well. The only problem now is that the PBCU can only keep the air coming in full blast. Once I push the automatic button the blower motor goes off. But at least right now I can get the system to work under full blast. It will just have to stay that way until I can purchase another one.

Thank you all for your help.

__________________
1999 Mercedes-Benz S600, 103K miles - garage queen
1988 Mercedes-Benz 560SEL, 89K miles - daily driver
2007 Hyundai Sonata Limited, 31K - daily driver