Hello and welcome to the Board !
Ecklers and a number of the other Corvette aftermarket parts suppliers seem to offer a variety of options- largely dictated by the size of your checkbook. Most revolve around an updated head-end - - digital, satellite w/cd-- a power amp and speakers including dash/kick panel replacements, a sub woofer and box in the rear between the fender wells.
I went that route through my local Best Buy (electronics, computers, TV's, etc) and did reasonably well in the past.
I recently went into a real "pro-sound" store that does home theater, surround sound and met a tech that offered to equip this c-3 with an upgraded system that looks stock - -he'll do the work on his off-hours so I know it'll function better (and sooner) than the one's I've attempted on my own.
In the Corvette's small cockpit- even with the tops off- we're gonna boogie-oogie-oogie `til ya just can't boogie no more ! Good luck and have fun on your project - -let us know how it turns out !
thanks
CQRT in AZ