Speaking to myself at this point, but I'll add the final chapter for the sake of future searches.

Traced the wires running to front and as I kinda suspected, they terminated in an aftermarket connector (4-pin molex type) under the driver's foot well kick panel. Must have been a brake controller of some sort in there but it's gone now. The main thing is, I don't have any live wires floating about the car. I'll remove the wiring one day, if I ever pull the door sill trims and/or carpets; until then it can all stay there.

Used the trailer a couple of times, including taking it to it's bi-annual safety check, which it passed, including all electrical device operations. Phew!

End of story...

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Wayne
2005 BMW X5 3.0d (b 02/05)
2001 BMW F650GS Dakar (b 06/01)