It looks like a green wire with a yellow stripe is what is jumped - can't really tell from your picture. If so... My best guess would be for an aftermarket remote start or alarm system that is (or was) installed. Just about every 1980+ Ford I've worked on, green wires with yellow stripes are constant 12V, whereas yellow with a black stripe are ignition switched 12V.
If this is the case, the location doesn't make sense as there are plenty of constant 12V sources under the dash. But never, ever underestimate the stupidity of a novice stereo equipment installer.
I'd say put a meter on it and go from there.
