he may not be able to go that route anymore since his wiring is now hacked up.
if your friend left enough line on the connector side i would really suggest connecting it all back together and going out and getting the GMCO wiring harness, its a little pricey at $100 but its the 1 you need.
if you get the harness you would have that stereo done and installed in about 10 minutes. the harnesses are idiot proof all you gotta do is match up the colors and plug it in