I used to have a tow bar fitted on my e46 touring. It wasn't the factory or dealer fitted option, both were very expensive. I got mine from Towsure and it was a removable system, used a pin and a R clip. Used to take about 30 seconds to fit and could not be seen when not used. Very useful for the kart and motorbike trailers. I have a towbar on my other car at the moment so won't be putting one on the 335d (although it would make a much better towcar), but something will have to be sorted out when I change that car.
Previously: 2007 335D MSport E90 Le Mans Blue, 2004 330D A Sport Silver Grey, 2002 330D A Sport Saphire Black, 2001 320D SE Touring Titanium Silver, 1998 M3 Evo Techno Violet, 1996 328i Sport Montreal Blue, 1994 325i Coupe Red, 1989 325i Sport 2 door Dolphin Grey,1986 325i SE 4 door Lachs