that's a DOH!! diode. in other words it's supposed to blow the fuse when you hook up the power to the radio backwards, prompting you to recheck your wiring. with the diode blown open i can tell you two things, 1) the power was hooked up backwards without an inline fuse, and 2) once the diode burned open it couldn't protect the radio's transistors from full reverse voltage, and something is likely damaged inside the radio, like audio output transistors......