To remove your head unis follow this guide, its very helpful:

Car Stereo Removal and Installation - Pontiac Grand Prix '92,93,94,95,96 - BOSE Car Stereo, Speaker / Amp Repair

About the dash kit, this whole kit will be also extremely helpful:

96-03 Pontiac Grand Prix RADIO DASH INSTALL KIT COMBO - eBay (item 130364804728 end time Mar-07-10 08:12:19 PST)

Hope this info can help you....