should be fuse 34 (15A) in the fuse holder at the end of the dash. It will almost certainly be the amp itself that has partly failed due to the usual corrosion problem with the pins of one or more of the amplifier chips. Best thing u can do is strip the amp down and clean the legs of the chips, i would also install a plastic guard infromt of the amp to protect it from water coming in behind the passenger light.
cheers
Steve