MB made a frame hitch for the 115, and I assume for everything else, too. Might cost three arms and a leg, but worth checking out, at least for design and attaching points.

Don't use a bumper hitch, you DON'T want to have to buy a new bumper, they cost the earth!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!