Is your car already equipped with a trailer hitch? If not, and I presume it isn't since the hitch itself is plated with the capacity, perhaps you could look into the 'approved' unit for your model/year and get the data from them. I once had a hitch from an older 116 that I sold here on this forum. It was made in Germany for the car, but the name escapes me. Anyway, perhaps this might help
http://www.etrailer.com/productdetail.asp?partno=24558&vehicleid=1770&yearfrom=1990&h=e