Recommended 4-Pole & 7-Way Trailer Wiring Harness for 2009 BMW 335d  

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Expert Reply:

The easiest 4-pole wiring harness installation will come from the no-splicing-required ZCI Circuit Protected Vehicle Wiring Harness # 119250KIT. This 4-pole harness installs simply by clamping around the Bimmer's taillight wires; you do not have to cut or splice anything in the vehicle. This harness does cost more than the hard-wired type like # C59496, but the installation is non-permanent and the harness can easily be transferred to another vehicle in the future. Something to consider.

If your trailer requires a 5-pole type harness, which can include a reverse light circuit for trailer backup lights or for activating the reverse lockout of a trailer coupler, you can add a 4-pole-to-5-pole adapter # 20036. This plugs in to the 4-pole and provides a blue wire for connection to the car's reverse light circuit.

If you will be towing a trailer that has electric drum brakes then you'll probably need to convert the 4-pole to a 7-way type RV style connector like the one pictured in the linked photo. This RV-style 7-way carries all of the same lighting signals that are on the 4-pole and 5-pole harnesses, and adds additional circuits for a brake controller and for 12V trailer power. To convert a 4-pole to a 7-way you will need our kit # ETBC7 which has all of the parts needed.

Electric trailer brakes require a brake controller like the popular Prodigy P2, part # 90885, which can be installed along with the ETBC7. No other items are needed apart from the 4-pole, ETBC7 and the controller unit itself.

I linked two articles on the ETBC7 kit and on brake controllers in general. Articles on towing basics are linked too.

expert reply by: Adam R

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