if you had a different year car, id recommend getting a replacement from a junk yard. if i can recall correctly a 2005 Chevrolet's radio is vin locked, so you'd have to take the junk yard radio to the dealer to unlock it before installing it. who knows what they'd charge for that. if it was me I'd install a decent alpine or pioneer.
Is it a bose system? the $99 adaptor is used to cure issues that can be experienced when disconnecting something from a vehicle's canbus system. gm had a press release saying that removing the factory radio in a gm car could cause airbag failure. I dont know if I believe it, but it is possible i guess.
There is an adaptor that does the bose system as well, but it requires a radio with front and rear outputs to function- so keep that in mind when you're looking.