I dont mean to tease anyone, but if your really want to do this conversion you can go ahead and find a donor car. 1980 is the best choice because of the manual A/C control. (Earlier 123's have the manual A/C controls but the evap/ heater box flaps are not as controllable and the harness is different because they used the York compressor).Start by removing the evap/heater box and then the wiring harness for the A/C system. Thankfully this wiring harness is a separate harness from the main one. There is only a couple of wires you will have to cut, one going to the gauge panel and one to the hazard switch. Make sure you label all the wires to be sure where they go. Your next step will be to rebuild the evap/heater box. This entails a complete dissasembly of the unit to replace all of the foam seals on the flaps. I have now taken apart four units and on everyone the foam seals where totally shot. This is a good time to also replace the vaccum pods as well. Once you do this it will function as it did when new.

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a few mb diesels