I am interested in installing an after-market stereo in my 2007 Trailblazer 3SS. Has anyone here done that? According to crutchfield, there is a part necessary to keep the On-Star and door-chimes working. From what I can gather, the part (http://tinyurl.com/33ojgf) takes the pre-amp output from the head unit, adds the On-star, and then outputs via the factory harness to the Bose sound system. Does anyone have any experience with this part (or one like it)?