I have a '85 300D that I resurrected the a/c system in. I replaced the compressor, dryer and pushbutton unit with new parts in addition to the vacuum actuator that opens/closes the "recirc" door behind the glovebox. Everything works as it should EXCEPT if I just want the vent fan on when the "off" pushbutton is mashed I have to mash any button EXCEPT the vent fan pushbutton and then I mash the vent fan button and it works fine. Speed varies based on temp setting, etc... If I go from off to vent fan pushbutton it will work for a few minutes and then turn off. I just live with it and mash one of the other buttons and then the vent pushbutton. The reason I had to replace the pushbutton unit is because it was not putting any power to the protection circuit starting at the evaporator freeze protection switch then to the pressure switch on the dryer and then to the Klima relay. There is a relay underneath the glove box on the right hand side that has a fuse on top of it. You may want to try to check it out too. My guess is that you need to replace the pushbutton unit.
I have found that the best troubleshooting manual for this car is the Mercedes-Benz manual titled "Electrical Troubleshooting Manual". It is around $35 from the 1-800 Mercedes number.

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Jim