Here's the link I was talking about earlier:
http://www.gm-trucks.com/forums/lofi...hp?t28030.html
They did a 6-disc slave install at the same time (a pretty nice setup) and the Nav looks like an older unit from a Trailblazer - the newer one is more flat. Either way, it might be worth a look, since a lot of the eBay units for sale seem to be this one.
Good luck with the install - definitely post how it turns out!