Unfortunately that only works for trailers that either have no brakes or have surge brakes. Electric brakes require a 6- or 7-pin plug.

Here's the Valley part for the 7-round to 7-round adapter:
7-Way to 7-Way Trailer Adapter for VW, Audi and Porsche Towing Trailers with LED Lights Valley Wiring V39010

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'04 E53, 4.4 Sport
'97 E39 528i
'86 911 Carrera, track car
'96 BMW R1100R