You might have a vacuum problem. There is a compressor vacuum switch that keeps the compressor from coming on till the engine is running. It is bypassed by the defrost position. That is the basis of my suggestion.

A wiring diagram is imperative here. Got a good one at work if you still need it Monday.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician