AXXESS GMOS-04 Replacement Interface for GM Vehicles - eBay (item 310213438174 end time Apr-20-10 10:31:33 PDT) $35 wiring harness.
Eclipse CD1200G - Car Stereo Head Unit In-Dash Receiver - Sonic Electronix cheapest double din head unit, or if you want ipod capability then just buy this Eclipse CD1200G+IPC-009 (cd1200g_ipc009) - Car Stereo Head Unit In-Dash Receiver - Sonic Electronix.
those packages should include your mounting hardware and an antenna adapter. that's everything you need for about 150 bucks. It's cheap and I would never do it personally, but for your needs this'll work.
