regular GM or GM/Bose - either way it's lousy!
best bet is to yank the entire system out and replace everything.
Alpine, Clarion, hi-end Pioneer or Sony ES are all great choices for a head unit.
than get the best quality speakers you can afford - it won't matter what anythng else in the system is if you put in cheap speakers. My preferences on speakers are MB Quart, JL Audio, Focal, or Boston Accostics. Every speaker sounds different and the only way to tell what you like to to listen to them and than decide.
Personally, I see absolutely no need for rear speakers in a car (except for subwoofers), especially a small car like a Vette. An excellent sounding and fairly simple system can be done by putting in a new head unit and run the RCA leads from the back of the radio to a 4-channel amp with a built-in crossover. from the front channels of the amp run the new front speakers and from the rear channels of the amp you can run one or two small subwoofers. 8" or 10" subs will be more than plenty and in a sealed enclosure the subbox can be pretty small depending on the model subwoofer. for subs, JL Audio are is my first choice by a large margin.
This would be a simple and easy system that would sound very good but it can also be easily expanded upon if you wanted to go bigger either now or at a later date.
BTW, if you like the Clarion headunits (their higher end units are very good) try Crutchfield)
