I got my Kenwood DNX5120 in-dash nav system installed in my 08 Silverado. I don't have the Bose system in it. I got a Scosche GM21SR wiring harness adapter off of E-Bay for $37 and it was a fairly easy install. All the wires matched up color for color to the Kenwood harness. I removed the stock stereo and plugged in the Kenwood with the GM21SR attached and everything seems to work just fine. I talked to OnStar to make sure that still worked and all the warning chimes seem to be operating. Only thing I don't have now is the steering wheel controls, but that's minor since I haven't had those in most of my previous cars anyway. And my XM, but I have a lifetime subscription to Sirius... so rather have that hooked up anyway.

So, just thought I'd share that it's not anything to really be afraid of doing, even if you want to keep your OnStar. There was even plenty of room inside the dash for the "box" that the interface has.