On the 300D you should squeeze the heater line to restrict it and be sure that the water isn't flowing. Otherwise it sounds like a refrigeration problem and I am not so good at the parts changing strategy of auto diagnosis. It really don't work well with the refrigeration.
On the later car there are enough info at the compressor relay to wire a dumb relay to the compressor on signal which will allow cycling. There is no reasonable way to just bypass the compressor speed signal.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician