mach 460 problem

okay i just bought a 1999 ford mustang convertable cobra. when i bought the car the passangers side speakers where very low i was told it was the head unit so i brought it to a sound shop they installed a new head unit. well the problem did not go away . I have read about the amps and other problems going on with this system anyone have any ideas it is all speakers on passanger side they come one but volume is so low sounds like someone wispering.