Welcome here! I am fairly new here too. I have an 83 300D, not sure if it is the same as yours. I had the same problem, the fan was not working at all with the button. Check under hood for blown fuse, mine was completely melted. Changed the fuse and voila the fan works again!

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1983 300D 292,000 miles
2002 VW Jetta TDI 153,000 miles

Got Diesel?????