As Towx says, you need the entire kit with the trailer wiring box, the harness, and so on.

However, if you already have that kit, and are just missing the adaptor that you attached a picture of, that goes from round to flat 4 (for a small trailer) then any trailer supply shop, or U-Haul, sell them for a few dollars.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue