Trailer control modules show up on eBay pretty frequently. I paid about $80 for mine back in January. The wiring harnesses are a little harder to come by. There's a Westfalia part# you can look for in addition to the BMW part. (Don't have it handy, but if you search Westfalia you'll find it). Making your own harness wouldn't be that big of a project. I started trying to find the OEM or equivalent for just the connector, but gave up that search when I found a new Westfalia one on eBay for $25.

Whatever you decide, I wouldn't be without the BMW (or equivalent Range Rover) control module. While some report that the generic wiring kits work OK, the electronics in our cars are fragile enough without adding something non-standard.... My $.02

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'04 E53 X5 4.4i
'05 Mazdaspeed MX-5
'00 Saab 9-3 SE Cabrio