my back speakers just stopped working

no idea what caused it but those are the wires i tap into to run my subwoofer. so i took apart my whole sub system before i realized it was the rear speakers signal that was shot. not the speakers themselves (because my subs don’t work either). i’m still getting 7 volts when i put either wire on positive and the negative to the frame…should that be closer to 12 volts? i looked at some fuses (and lost the one labeled “amp”) but no obvious difference. could wiring just “go bad” , on both sides at once? is that a head unit failure?

 

i mean it’s all aftermarket but i had to tap into the audio somehow, right? i just didn’t tap into directly behind the head unit. so i took the door speaker wires put them through a line out converter (to get rca) to a bass reconstruction device (looks like a small amp) to an amp (of unknown strength) out to my subwoofer. It’s spontaneously started working again , so problem solved but mystery remains