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There are a lot of writeups on this at TGO. Essentially, you need to jack the passenger front side of the car, remove the wheel and go in through the wheel well to get at the power antenna. There is no connector for the antenna, so you will have to make your own connector or solder back a new one. Try GM Parts Direct: Your direct source for Genuine GM Parts for an OEM replacement.
Oh and note that the GM OEM replacement sold these days (88891017) is far away from the units fitted to these cars as it's a generic replacement GM use for all older cars .... make sure you see the replacement and compare to your old one if you decide not to rebuild yours.
I would rebuild your existing antenna, I have done it twice, not that hard. I also have a gta with the oem replacement and it rattles around in there when the car starts and is clearly not the original as there is a chrome collar on the fender.
Also the rebuild kit will probably be less monty than the replacement antenna.