Thanks for the tips guys!
The cheapest harness I could find is $450 on eBay.
I dropped by the stealer this morning an asked the same question. His response was killing me: "Oh no- you can't do that! It's not prewired? First you'll have to get the harness and take the car apart to put it in and you don't want to deal with that. Would you like to take a look around our showroom or CPO lot to see if there's any X5s with the Factory Navigation that you'd like to purchase instead?"
I think this is something I could live without.