[oldskool]

It sounds like whoever hacked your wiring up used the yellow/red which is the wire that becomes live when your ignition is on to power up the cigarette lighter

This wire is important to a stereo as if you didnt have a switched live and just connected your two power wires on the stereo end to 12v constant your stereo would not switch on and off with ignition and could possibly flatten the battery etc

I would splice a wire off that yellow/red from the car loom and it should connect to the one on the sony stereo marked ignition live

Tbh its a bit tricky to say what you should do if you dont have a test meter/ lamp or bulb etc as its easily possible to damage a headunit by putting 12v in the wrong wire so if in doubt defo dont just connect things up if unsure