We must know what year model or series this car is. There are several different 300D's and it's impossible for us to read your mind.
If it's a late 123, 81 to 85, the problem is most likely the monovalve.
If it's a 124 car, the problem is most likely the pushbutton control unit.
Let us know what model and we will give you more details to help you properly troubleshoot before wasting money on unneeded parts.
Have a nice day,