The biggest problem with having someone not familiar with an X5 Hall Effect wiring harness is that you end up with electrical problems. You have to know where to tap for the correct signal since one wire may be shared by multiple electrical circuits, thus the Hall Effect. Per the factory, there are only two acceptable ways to connect brake lights- 1) with the factory harness adapter or 2) by tapping the correct wire in the Light Control Module which is behind the right front kickpanel (basically by the passenger's right ankle). Anything else is a crapshoot. Please do yourself a favor and get the factory harness. The $$ you spend today will save you the horror of having to replace a melted harness later. It really is plug and play.

Also- did your mechanic prime, paint and caulk all the holes he had to drill in your body to install the Curt? There's a lot of electronics in that spare tire well that you don't want getting wet from a leak, and you certainly don't want rust in an area you're relying on to support the hitch. Glad he used bigger hardware, the stuff that comes with the Curt looks cheap. Hopefully he used at least grade 8.8 if not 10.9 hardware. As I recall all the bolts with the factory hitch are grade 10.9, but it's been close to 4 years since I installed mine so maybe it was 8.8.

Hope you get your harness squared away.

__________________
'04 E53, 4.4 Sport
'97 E39 528i
'86 911 Carrera, track car
'96 BMW R1100R