I installed a touchscreen navigation system (including the Bose Luxury amp) from an Escalade. Because it was made for another GM line of similar vintage, it communicates on the existing Class 2 Data Bus. By doing this I get what I feel is very decent sound quality and navigation. It controls my Bonneville CDX, works with OnStar, retains my door chimes, works with my factory XM receiver and functions correctly with my steering wheel controls. The down side is that this vintage of radio will not play DVD'* (although it does use a DVD map disc). Here are my threads on installing it so you can see.
In Dash Navigation Unit.
Navigation - Update.
You can see the Dock-N-Lock connector in some of the photos. Dock-N-Lock is the name of the style of connector GM used. Yes, as Black_Friday stated, the radio just slides into the bay and "docks" with the connectors in the back. The tabs on the side of the radio lock it in place, no screws required. These connectors can be difficult to work with because they are mounted in the rear of the bay and there is very little room to work. These were used on a few years of the Aurora, Bonneville, LeSabre and IIRC Park Avenue. Overall, they are not common and therefore there hasn't been a lot of good support for aftermarket interfaces.
Here'* a photo of the back of the stock radio.