I installed a Pioneer DEH-P5000UB in our 98 GT.
Got the install stuff from Crutchfield:
003GMK432X Pontiac Kit 1995-up, S/E 19.99
120701858 GM Wire Harness receiver, 1985-up 9.99
12040GM10 GM Antenna Adapter Motorola M to OEM antenna F 9.99
127SWIPS PAC SWI-PS - Pioneer Steering Wheel Radio Control Interface 74.99
130CDIU50 Pioneer CD-iU50 USB-to-iPod Adapter 29.99
I ran the USB cable into the console, where the iPod cable attaches. With the iPod connected, you can control the iPod from the stereo. Or, disconnect the iPod and you can control any USB storage device with a standard file system.
The install calls for the deck to protrude 1/2" out from the dash, but I used a sawzall to cut off the plastic rear support for the OEM radio and now it sits flush. No vibration, works great! Will post a pic as soon as the car is back from the tranny shop. Sounds pretty good with the stock speakers too.
The steering wheel adapter works great and was a breeze to install. Plug n Play, just follow the programming directions that come with it.