i would have thought thats the battery wire, wont turn on without this connected
their should be a plug to connect this into on the new harness, usually their are 2 red ones and maybe 1 or 2 yellow ones..
are you sure theirs no wires with a metal connector left on the harness of the new stereo, im sorry im not familiar with JVC as i only use Apline HU but i guess the wire harnesses should all be the same