Audio Systems: Range Rover 11 Speaker HK system., speaker stereo system, harmon kardon

Question
I just purchased a used 2001 Range Rover SE with the Harmon Kardon 11 speaker stereo system.  When the radio is on or a tape is playing (I also can not get the head unit to recognize factory 6 disc changer) the only area you here sound from is the subwoofer.  Each front door has its own amplifier and 3 speakers (tweeter, mid-range, driver) while each rear door has its own amplifier and 2 speakers.  The subwoofer is of course powered by its own amp.  Could the 4 amps that run the 10 speakers somehow be run in a series where one bad amplifier shuts the other three down?  I'm trying to fix this in the next few weeks before vacation.  All the local shops say that the ten speakers and subwoofer would be non-conpatible with an aftermarket unit so I'm trying to resurrect this system.  thanks for any help.

Answer
I'm not familiar with that exact system over here but its back to basics and what is it that stops an amp working and what would I be looking for? Well, are the speakers blown? It has been known, check them out with an ohm meter or on a known good source. Power is another one, so use a 12v bulb probe or a multi-meter set to 20vdc, check that when 'on' the amps have 12v battery (+30) permanent feed also is there 12v power on the 'amp' feed from the radio to turn the amps on?
Each amp should have 12v permanent power, 12v signal from head unit to turn it on and an earth as well as the RCA leads for the speaker input from the head unit and of course the speaker leads out to the speakers. As the fault is on all but one of the amps I would be looking for a common fault. Failing that it’s a possible head unit fault and off to the repairers with it.
As far as compatibility, if the new head unit has the same pre-outs at the back (4 x RCA + woofer) and a standard DIN connection I cannot see why not.
I hope that helps. Peter