Although many audiophiles pooh-pooh them, there are some good quality inline FM transmitters that need to be hardwired in. The only issue is that they usually come in 1/4" antenna and GM used the mini-antenna, so you need to adapter-up to the transmitter box, then adapter-back-down to the radio. I have my van set up like this and it works very well. Maybe the audio quality isn't show-ready, but it'* there and it doesn't hiss or crackle. I got all the stuff from crutchfield.
OTOH there is no OEM radio quite like what'* available in the aftermarket these days. USB MP3, MP3-CD, built-in iPod controls, bluetooth handsfree through the car'* speakers, etc are available in most sub-$300 head units. I have a Pioneer FH-P800BT in my Riv, and a PAC-SWI(PS) to keep the steering wheel controls alive. I love the features and setup.
The wired antenna adapter is something like this from crutchfield: