It could be a number of things. I would first check the connections for the amps mounted under the rear deck. Turn the system on and use a multimeter to see if you are getting 12v power at the main power lead and the turn on lead, check for bad grounds and make sure the connectors are clean and fully connected if you have 12 volts at those leads with the system on. Also check that all the factory harnesses are clean, undamaged and fully connecting.
One problem might be that the previous owner put in an aftermarket deck and connected the front and back speaker leads without using an adapter to lower the wattage and/or signal to the amps. In the factory Mach 460 system the speaker leads from the Mach 460 deck are low power signals that act as the input for the amps. So if they did that they blew the inputs on the amps, and you will need to replace the Mach 460 amps, or put in a aftermarket system.
