The only hitch that we have confirmed to fit the 1981 Mercedes 300TD Wagon is # C11805. The trailer hitch comes with the recommended ball mount, pin, and clip, and you can use ball # 19256 (1-7/8 inch diameter) or # 19258 (2 inch diameter). For trailer wiring, you would need Curt Powered Tail Light Converter with 4-Pole Flat Trailer Connector # C59236. I have included a link to the installation details for reference. view full answer...