02 Limited speaker problem

I just purchased a 2002 explorer limited V8 last week and noticed a problem with the speakers. I didn't really notice it when I took it for a test drive because I wasn't too concerned with the radio. Anyway, the speakers go from normal volume to really quiet. It does this when I go from ignition on (ACC position) to actually starting the vehicle. When the ignition is on (vehicle off) the volume works fine but as soon as you turn the vehicle on the speakers get really quiet and I have to turn the volume nearly all the way up to hear it at a decent level. I just replaced the front passenger speaker because it wasn't working. I tested the wires with another speaker before replacing it and the wiring worked fine. All of the speakers work fine now. I pulled out the 6 disc changer and checked the wiring in the back, at some point somebody changed/re-wired something because there's wires spliced together. I took the two wires that were together, cut them and reattached thinking maybe there was a bad connection there, no dice. I peeled back the the electrical tape as far as I could and didn't notice anything loose or not attached. When I do turn the vehicle on I can hear a spark like sound (loose wire type of sound). Short of tearing apart the dash is there anything else I can do or has anyone else had this problem with this model? Any help would be greatly appreciated. Thanks!