[hatzie]

Whatever you do don't butcher the vehicle harness by cutting off the GM plug. There's Class II serial data in that radio plug for the warning chimes among other things you don't want to have just floating loose in the dash behind the radio.

I used a pigtail from Metra. They're one of may manufacturers of these. There are several manufacturers that make vehicle specific pigtails to plug right into the OEM radio plug.
They cost somewhere between $6 and $15 US.
The pigtails plug into the GM radio plug and they come with fairly decent instructions on the bag for connecting the adapter pigtail to the aftermarket radio pigtail.
I usually solder and heatshrink the connections between the Metra and (Sony, JVC, Kenwood, Alpine, Blaupunkt, Grundig, Etc) pigtails and heatshrink over the ends of the un-used wires to make sure they don't make electrical contact with anything.
Once have your adapter harness made up it just plugs into the GM cab harness radio plug and away you go.