Be gentle when you take off the dash! Pull it off carefully. Adjust the steering wheel to it's lowest setting and push the turn signal stalk down. The metal clips that hold the dash on might come off and spring around the interior - careful!
Make sure you get a quality radio install kit, like from Metra. My '99 GP used Metra part no. 99-3320. You'll need an antenna adaptor which will adapt the aftermarket antenna to work with the GM antenna. Get a wiring harness adaptor too (Metra ABMW-1858), it really makes the install much easier than trying to match all those wires. Altogether it was about $40 in parts. Should take no more than 2.5 hrs.
~Steve