I replaced my OEM 6 disk changer with a Pioneer Avic-Z3 - Nav, DVD, 40 Gig drive for music, voice recognition and all the adapters (GM interface, steering wheel controls) for $500.
From what I've read, it blows away the OEM system. And I didn't have to cut one wire or trim anything nor change the amp - it plugged right in.
I did, however, lose the rear seat controls....but I sit in front
Z1's and Z2's are software upgradable to the Z3 - the software is "available".
Just a thought, you might want to think about aftermarket.
-Dave