Alex:
I've been using a Ultraguard Covercraft Car Cover for the past 6 months. It's a amazing how clean the car stays when I use it. The cover I purchased for the 500E was a custom fit ... and it fits tight. There is even a pocket for the hood ornament ... overall, great quality, and it can be washed (I've done this several times) in a standard large capacity washer/dryer.
I paid just under $300.00 for the cover ... outdoor type. Here is a link of the same cover that I purchased but configured for your 300D.
http://www.autoanything.com/carcover/product.asp?make=Mercedes-Benz&year=1983&model=300%20D&prID=168&submit=yes&cat=
Listen to Afshin, GET A CAR COVER!