Background: 2004 Max, BOSE system, Pioneer headunit

Problem: Everything started out fine after the Pioneer HU install. Recently though, the left front lower door speaker only comes on when I turn the volume up pretty high. I can then turn the volume back down and it will stay on. If I hit a bump it will sometimes go off. I can then turn the volume back up and it will come back on. I have taken the speaker out and hooked it up to another connection and it worked fine. I figure there's a short in the wire somewhere, but why does it come on when I crank it up? How am I going to figure out where the problem is?

(I've also posted this in the Audio/Electronics section)