I think I have the adapter your talking about. GMOS04 or something like that. I got it to install my Alpine head unit in my 01' Grand Am last year and it didn't work with my Monsoon. I now have a 2004 GP GT and Crutchfield says that it's supposed to use the same adapter but I'm pretty sure it's sending too much power to the factory amp. I need to find the amp (looked in the trunk between rear speakers) and turn down the inputs and I think it would work. So basically in a nut shell I couldn't get it to work with my Monsoon but it seems to almost work with my factory amp in my 04 GP. I think your best bet would be to get a head unit with OnStar inputs and replace the amp. You would lose your door chimes though. Monsoon all together just kinda sucks. :/
