You can get a great DD DVD/Nav headunit (Pioneer AVIC-D3) from best buy right now for $700, or on eBay (new and/or refurb) for around $600. Yes, you need a "chime module" to retain chimes and Onstar, but that can easily be found on eBay for under $70. Then you need a dash kit and antenna adapter (another $20). You can do away with factory chimes and onstar if you wish and go with a $15 harness. If you buy this unit at Crutchfield (the unit is $1000 minus a $200 Pioneer MIR until 3/1) will supply the basic harness, adapter and dash kit for you at no cost. That actually applies to any HU purchase at Crutch.
Regarding the install, unless it's different than the Imp, you pop the dash trim around the radio (held in by spring clips only) and unscrew the radio and HVAC controls (6 screws). Pull them out, separate the two units, and R&R the radio. AFAIK, there is no special took as Sint3k described...that was primarily a Ford radio thing. Google don't no jack

...although I DO rely on it more times than not, it's not right in this case.