I have an "A/C Delco" Power Antenna Kit. Everything comes with, but I'm not sure if it has a bracket (for the mount on the bottom).
The GM Part No. is: 88891017, and the Delco No. is: 88891017 and it's new / mint. Price is 175.00 / plus shipping. I bought it years ago ...
If that doesn't work, perhaps I could help you with a particular part as I have a few of the orig. "GM Style" to fix YOURS (now).
All this just FYI ;- ) Best.