I have an extra wire after reinstalling stock radio

I removed my aftermarket stereo head unit and in the progress of putting the stock head back in. I have all the wires spliced back together but I have an extra one that doesn't look like it hooks up to the wiring harness. I have looked through the wiring diagrams in the tech manual but can't find any reference to it. 4 wires go to the front speakers, one i have hooked up (cant remember where to right now), and the bigger black wire is the one I have no idea where it goes to.
Anyone have experience here? Its been 5 years since I had the dash opened up.
This is just the basic bottom end prius, nothing fancy from the factory.

 

That's what I was thinking but unless it was meant to just have a terminal attached to it and hooked to one of the bolt holding the stereo in place, I see no other place to put it.

 

When I installed the aftermarket head unit I spliced the wiring harness in the car to the wiring harness that plugs into the back of the aftermarket head unit. I reattach all the original wires. All the wires coming from the car are accounted for. This one big black wire coming from the connection that plugs into the stock stereo has no wires I can splice it to.
Looking back I wish I took a look at where everything was going to on the aftermarket stereo. I took a look at the scraps on the ground but can't find any reference to this big black wire.
The stock stereo is going back in because the car is going to be traded in. It has taken good care of us for the past few years but with the baby and the dogs it's just too small.

 

Well I don't know why it was there but it doesn't look like it was nessesary. Plugged the connections in without tge black wire and everything worked. The connection at the end of black wire that was in the plug didn't even look like the rest of them. I don't know why it was in there but everything is all good now.