Adding a 7-Way Trailer Connector to a 2005 BMW X3  

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If the trailer you plan to tow has a 7-Way round pole connector, you can install a matching connector on your 2005 BMW X3. But an adapter to go from a 4-Way flat to a 7-Way round pin is not available. Below I have explained how to install a 7-Way round pin connector on the vehicle, but it may be much easier to install a 7-Way blade connector on the trailer using # PK12706. You can use the wiring diagram I have linked to wire the 7-Way blade correctly. I have also included a video that should help.

You could then use a 4-Way to 7-Way adapter like # 37185 for just light functions on the trailer. To make it completely functional, installing a 4 and 7-Way Installation Kit, # ETBC7, would be best. I have included some FAQ and video links that explain the installation of an ETBC7.

To install a 7-Way Round on the vehicle, first, you will need to install the 4-Way wiring harness, # 119190KIT that you have referenced. In order to install a 7-Way round, you will also need Pollak Heavy-Duty, 7-Pole, Round Pin Trailer Wiring Socket, # PK11720, and Rubber Boot, # PK11761.

What you will need to do is cut off the 4-Way connector at the plug on the 119190KIT. Then, you will have 4 wire that will need to be attached to the back of the 7-Way round. The white wire will attach to the pin in the 12 oclock position. The brown wire will attach to the pin in the 2 oclock position. The Green wire will attach to the pin in the 4 oclock position. And the yellow wire attaches to the pin in the 8 oclock position.

There will be 3 pins left after you are done with the first 4 functions. These are used for 12 volt power to the trailer, electric brakes, and reverse lights. In order to utilize all the functions, you will need some additional wire, # 10-2-1, about 25 feet worth (enter 25 in the quantity box on the product page).

The 10-2-1 wire has 2 wires inside. Attach the black wire to the pin in the 10 oclock position and the white wire to the center pin. You will then need to route the wire under the vehicle, avoiding areas that may pinch or burn the wire, up to the engine compartment. Here you will separate the 2 wires. The white wire will need to be routed through the firewall and into the cab of the vehicle. The black wire attaches to the positive battery terminal via a 40 amp circuit breaker, # PK54540. Any access wire can be trimmed off and used later.

The white wire that you routed into the cab would connect to the blue wire of a brake controller for electric brake output. I recommend the Tekonsha P2, # 90885, because it is easy to set up and easy to use. The brake controller will require additional wiring after the blue wire is connected. Using any excess wire, you would connect one end to the brake controller white wire, route it through the firewall, and attach it to the negative battery terminal for ground.

For the black wire on the brake controller, again use any excess wire and attach it to the black wire. Route it through the firewall and attach it to the positive battery terminal via a 20-amp circuit breaker, # PK54520, or a 30-amp circuit breaker, # PK54530, according to the brake controller instructions (linked for reference).

The last brake controller connection is the red wire. Using a circuit tester like # PTW2993, find the wire on the brake switch, located above the brake pedal, that carries a signal ONLY when the brake pedal is pressed and attach the red wire to it.

The last connection is back at the 7-Way for reverse lights. Using excess wire and the circuit tester, attach the wire to the remaining pin on the back of the connector and route it to one of the vehicle tail lights. Use the circuit tester to find the reverse light wire behind the taillight. Make the connection and the installation will be complete.

To mount the new 7-Way round pin connector, you can use Bracket # PK11771.

expert reply by: Michael H

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