The large A/C compressor control relay "Klima" uses numerous inputs to justify the A/C clutch on/off decision.
One of those inputs is the control signal for the compressor from the pushbutton controller. The necessary by-pass would use that signal alone to control the relay. The whole relay could be rewired using a simple relay and the necessay circuits or the "Klima" relay can be disected. I have altered a number of these relays to be tools. The other signals can be over rided with a simple wire soldered over that section of the relay.
You will want to by-pass the engine safety controls, not the A/C controls. Running the relay straight from the ignition only will cause the system to freeze up. All the A/C needs are handled by using the control signal to activate the relay.
Bosch Master, ASE Master, L1
33 years MB technician