I haven't worked on one of those, but the most common cause of such problems is bad wiring between the body and the door where it flexes, followed by burnt out speakers. Does opening and closing the door, or wiggling the bundle of wires to the door, make it go on and off? Does thwapping the door over the speaker make it go on and off? Does poking something through the grille and pushing on the speaker cone make it go on and off? If it's not the door flex wiring or the speaker, then look into the radio itself and the rest of the harness wiring.