83 K10 Wiring Help

[noclrnce]

Well the Sony radio the previous owner installed doesnt work. I checked and it doesnt look like they hooked the wiring up. The factory plugs were cut off and the wiring is just there but not hooked to anything. I planned on running new speaker wires so that doesnt matter much. The part Im not sure on is there are only 3 wires left.....yellow, black, and gray. The black is ground obviously. Im guessing the yellows go together. Everything I find says gray is illumination. Where do I connect the orange wire?