Finally wired my E53 back up for trailer lights. When I bought my E70, I planned to sell my E53, so I sold the factory trailer hitch and wiring harness. Plans changed and I kept the E53 around, so I put a Curt hitch back on it so I could haul bikes at least. Then on occasion I really wished I had lights so I could pull a small trailer.

No coding is required, so all I had to do was buy a $60 module off Ebay, the plastic plug and the individual pins are available from FCP, and lastly a generic pigtail (I only made a 4-pin hookup for now). It's a really clean install doing it this way. The blank plastic plug and pins were about $10 and a Curt pigtail was about $6 on Amazon. I crimped on the pins to the individual wires and then soldered each for durability.

Plug the module in behind the battery, plug in my homemade, but good-as-factory harness, and I have lights!

AM.

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E70 2010 3.0 M57 AT Titanium Silver with Black Leather
E53 2004 3.0 M54 AT Toledo Blue with Gray Leather
E83 2008 3.0 N52 AT Silver Gray Metallic with Black Vinyl
E46 1999 2.8 M52TU 5MT Black with Tan Leather 250K+ miles (Sold, but not forgotten)