Is there a ghost in my stereo???

Hi folks, I have some eerie goings on with my 1999 TD5 ES original spec stereo. It is the Harmon Kardon set up.
Recently it has taken to turning itself up and down at will, it also fades in and out from front to rear - doubling the volume.
The remote stero adjusters in the rear do not work at all.
I took the rear tailgate speaker cover off to find two speakers - one of which appears to have been eaten by something and no Amp.
Should there an amp of some sort in the rear?
Any input at all on this matter would be greatly appreciated as I am completely unaware when it comes to electronics/ wiring etc.
Many thanks,Will .

for the volume/fader mystery do you have steering wheel controls?

something similar was happening to my p38 with HK setup but once i unplugged the controls all was good

Hi, yes I do have steering whel controls but I think they only control only mode and volume.
When the stereo turns itself up the sound quality also increases dramatically if that is anything to go by.
Thanks,Will .