I bet you have an aftermarket stereo. Its the first thing I check when someone brings me their car with a similar issue. Most the time its simply from the blue amp wire that then dont cap because they dont think about it. But I have seen the two rear speakers ran ran off the same output as the two front and not capped.
If it isnt that then your just gunna have to look at all the ends. Wire isnt going to short out in the middle of the car unless someone is an idiot and moved the wiring. Check behind the fuse box under the dash, behind the cigarette lighter, trunk light, your main grounds, maybe even your main in line fuses