If you have the Mach system there are 3 factory amps. One drives the tweeters, and the other two drive the 4 woofers. If the tweeters are the only speakers playing I would check fuses. I think the fuses for the amps are in the fuse box under the hood, not the one under the dash. If you cant find any blown fuses I would assume that there is a problem the the factory amps and you will need to start repalcing equipment.