Where is the speaker relay located in 1991 Corvette convertible?

There isn't a single "speaker relay" in a 1991 Corvette convertible (or most cars, for that matter). Each speaker, or group of speakers (depending on the factory audio system's configuration), is usually controlled directly by the amplifier or the head unit itself. There's no intermediary relay. If you're having sound issues, the problem is more likely in the head unit, wiring, amplifier (if equipped), or the speakers themselves.