Guys: yes, the CCU puts a ground on pin 10 through the low pressure switch. With one side of the relay coil on Klima pin 5 for power and the other side on Klima pin 10, the relay will operate when the CCU is turned on and outputs the ground to pin 10. And if freon pressure is low, the low pressure switch will break this ground connection and shut off the compressor.
I've read some Mercedes material that says the compressor runs all the time when the CCU it is set for heat OR A/C and that the temperature of the air to the interior of the car is adjusted automatically by adjusting the amount of heat to mix with the air flow. This would explain my compressor being off when the "0" button is pressed but is on with every other setting. I didn't observe the behavior of the compressor with the original Klima installed to confirm this to be correct. I will. Anyone else observe this?
I am now researching the diesel kickdown behavior that turns the compressor off under hard acceleration that MXFrank mentions.