John
I spent a number of years in the car audio industry. I was a sales rep for many of the higher-end car audio manufacturers and also managed a couple of high end car audio shops. While managing the shops we did very simple radio installations up to $100,000 custom competition system. Based on that i'd like to think I know at least a little bit about your situation.
To put it as honestly and bluntly as possible, although GM charged a good premuim on the bose systems in their cars, the Bose system really wasn't all that great of a system. One, it never sounded all that good. and two, upgrading it was a real b**ch.
If a customer came into our shops and needed repairs or upgrades to their Bose system we almost always recommended just ripping out the system and starting from scratch.
Now understand, although we were a shop and wanted to sell systems, our recommendation to replace the Bose system was NOT just to sell them our equipment instead. We also did in-house repairs and with the fairly low margin of mark-up on the equipment we made a LOT more profit on repairs than on selling new equipment, but we knew that for the money that was going to be needed to be spent on either repair of the Bose system OR a new replacement aftermarket system the customer was usually better off replacing the Bose with aftermarket and it would be more reliable and sound MUCH better.
I don't know what you consider "big bucks" as you say in your post, but a good shop will charge in the range of $40-$75/hr for installation. This allows for a profession and correct installation in your car from experienced installers that know what they are doing and will do the job correctly and it will look, fit, and sound right and they will warranty their work.
If you are used to doing things yourself on your own car it not something that is overly difficult to do though. If you have never done it it will take you some time but it's something that if you are not scared to get in there and do it, try it yourself.
If you want to post back and give me an idea of what you would be looking for in terms of an overall goal for the sound system I could help with a few recommendations on ideas to consider. Since the interiors of Vettes are small, it really doesn't take a lot to get a good amount of high quality sound out of your car.
Barry