are your stereos hooked up through the CD player? or through an amplifier? if its through the stereo then i'd say its a definite connection problem; but if you've replaced the stereo and still the same problem, im going to assume that your speakers are hooked up through an amplifier. if this is so, go out and buy a wire tester, its a little screw driver looking thing that has a tip on the and a light in the middle, im sure you know what im talking about. anyways, use that to try and test your connections, first connect it to your power to see if your getting sufficient amount to the amplifier, next hook that ground section up to it and test the power again to make sure your getting a sufficient ground connection, after that i'd test each and every one of your speakers, and if you can't find a solution i'd say replace the amplifier. if you blew a fuse, then there was a short with a connection somewhere, with amps if the short is still going on the amp will turn on but will usualy have a red light instead of a green one, or something of that sort.