I got this e-mail a little while back about this problem, so I thought I would share it with everyone in case someone else comes across this problem. The link below has a nice write up about putting in a radio in a 2000+ bonneville and also tackles the AM radio problem that I had.
Much thanks to this fellow for helping me trouble shoot my problem and I hope this helps someone else
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Hey, I had the same problem with my 2000 when I put a Sony in last year (http://homepage.mac.com/markhooton/radio_web.html
). A dollar to a dougnut that you will need to connect to your antenna booster...(ceiling on drivers side just forward of the rear window)......