That's exactly how it was with the 300TD. I'd checked the PBCU, temp controller, fan controller, resistor pack and installed all new fuses - and still no response from anything when I pressed buttons. I decided to pull the PBCU out and check each pin on the connectors to see if it was getting power at all. I had left the ignition switch in the "on" position and as I was pulling the connectors off of the sides of the PBCU, the loose/corroded pins made contact and the vacuum switchover valves clicked and the blower kicked on. I used a small piece of rolled-up sand paper to clean the contacts in the two connectors and pliers to squeeze a few of the loose contacts back together. That fixed that problem, but sometimes the ACC would shut off when I hit a bump while driving. I could get it to come back on by shaking the key a bit, so that was the worn out ignition switch. I just finished replacing the switch last week and so far everything seems to be working normally.