What Wiring, 4 Flat, 7 Way, and Brake Controller do I Need for My 2021 BMW X5?
Updated 06/23/2026 | Published 06/13/2026
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Question:
I have a 2021 x5 m50i that i installed a hitch and i need a wiring kit for it. Im not towing a full trailer. I tow my mazdaspeed3 with just a small basic dolly. What would be a ideal wiring kit to use for my small dolly. Im just need ur advise so i can purchase it from.u guys. Thank you.
asked by: Joel E
Expert Reply:
Hello Joel,
Thanks for writing in.
If your tow dolly uses a standard 4-way flat connector, then all you'll need on your 2021 BMW X5 is the Curt Powered Tail Light Converter with 4-Pole Flat Trailer Connector # C59236. This is a universal powered converter that installs by connecting to the BMW's tail light circuits, drawing power directly from the battery, and routing the 4-way connector to the rear of the vehicle. Once installed, it will provide the running lights, brake lights, and turn signals needed for the tow dolly.
If your tow dolly is equipped with brakes and requires a 7-way connector, you can build on the # C59236 by adding the Universal Installation Kit for Trailer Brake Controller - 7-Way RV and 4-Way Flat - 10 Gauge Wires # ETBC7. The # ETBC7 uses the 4-way from the Curt converter as its foundation and adds the wiring needed for a 7-way connector, including the circuits required for a trailer brake controller and auxiliary power functions.
If you do have brakes, you will also need a brake controller. I really like the Redarc Tow-Pro Elite Brake Controller - Dash Knob - Off-Road Mode - Up to 3 Axles - Proportional # RED44FR. This brake controller is easy to operate, and works great! I really like how the knob is accessible for easy adjusting, or in case you need that manual override to fix sway or in an emergency situation. I attached an image so you can see what I am referring to.
Most likely you just need the 4 flat wiring with how you described the tow dolly, but just in case I wanted to include the other parts as well.
So where are you towing your Mazdaspeed3 to, are you more into the shows, or the racing, or both?
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Product Page this Question was Asked From
Curt Powered Tail Light Converter with 4-Pole Flat Trailer Connector
- Custom Fit Vehicle Wiring
- Trailer Hitch Wiring
- Vehicle End Connector
- Powered Converter
- Universal Fit
- 4 Flat
- Plug and Lead
- Splice-In
- CURT
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Joel E.
6/23/2026
Thanks for all the info. I ordered the basic kit. Installation was pretty smooth. No shows or races, just like to be able to tow it whenever it breaks.