sorry my bad, 1991 4runner.
I found that harness under the left side rear seat. it came from under the carpet and I am thinking the previous owner pull it out from somewhere. I just have no idea where it goes

I pulled out the slack and that is where it ends up in the trunk. the other end disappears under the carpet towards the front of vehicle

 

Well it looks disconnected, if so it's not that important if your still driving. If it's speakers it will be either 2 wires for unpowered rears, or iirc 4-6 for amplified but those run in the Wall and bundle with other stuff.