Because of the totally outrageous cost of the X5 trailer hitch setup from BMW (2000-2500 installed, per the dealer) I decided to DIY with non-OEM parts. Execuhitch receiver (lots of reviews of them out there) and a universal lighting kit from eTrailer. The Execuhitch was about 700 dollars installed, the lighting I did myself (and I'll describe below, pretty easy), with a 74 dollar trailer module. Total cost was around 800 bucks.
First off, this is the kit I used:
http://www.etrailer.com/Custom-Fit-V...id=20141170285
Kit included just about everything I needed; I still need to get one more butt connector, but, that's just my particular install. I put the module in the drivers side compartment, nice place for it to sit there in the foam insert, almost like it was made for it. There's also a very nice spot for a ground right next to where the module sits, easy to get to, and a very solid connection (it's just a bolt welded to the body with a nut on it).
Now, the thing everyone wants to know; how do you wire it?
The wires you need on the drivers tail light (BMW side) are:
Black/White (blinker)
Purple (running lights)
and on the passenger side:
Black/Purple (blinker)
And then, to the module, it goes like this:
Module wire X5 wire
____________________________________
Yellow -> Right side (black/white)
Brown -> Right side (purple)
Red -> GROUND (same screw as the white wire)
Green -> Left side (black/purple)
White -> GROUND
Works great, cost 1/10th the dealer trailer lighting package, and took about 2 hours to install (would take you less now that you know what wires you need).
Now, all that said, if I was going to trailer a lot, I'd pony up for the dealer installed wiring. It includes coding (so your park sensors don't go nuts and you have the trailer zoom feature) and it looks better in use; if you use this module, when you're trailering, you'll have a wire hanging out the back of your tailgate. A little "ghetto" for a 80K truck. But, if you; like me, only pull a trailer every now and then (I have a small trailer that I pull to Lowes/Home Depot to pick up lumber/plants/mulch/etc) then this is good solution that will save you close to 1000 dollars and work just fine.
BTW, for anyone who trailers, I'd suggest you write a nasty e-mail to BMW NA (as I have) letting them know your displeasure at the trailer hitch not being a factor option in the US. Not only is it ridiculous expensive have the dealer add it, it also CANNOT BE RESIDUALIZED in a lease! That's just ridiculous, by the time you get done with it, you're going to wind up paying 50-75 dollars more a MONTH to have trailer hitch on your car if you go down the dealer route and lease the vehicle. That's just insanity, it's a hunk of metal and a few wires, and, even more damning, none of the other lux brands do this (on my previous ML, the factory hitch was 500 bucks and could be residualized, turned into about 7-8 bucks a month, IIRC). BMW needs to get their heads screwed on straight about this, I know a lot of people with an X5 will never tow, but quite a few will, and this craziness is just unacceptable for a truck (especially one that comes in diesel and has 4WD, it's a very good tow vehicle right out the factory). <rant over>