So far as your description leads me to believe, the installer tried to be clever and merge together a 3.1 system (front) with a 2.1 system (rear). It seems that job one is to construct a wiring diagram of this poor fellow's setup.

If I had to hazard a guess, one of the wire runs flaked out or got damaged, and you'll end up offering your client a variety of choices, ranging from re-pulling wires, to "normal" speakers that a standard amp can drive, to some sort of soundbar (as my parents have ultimately done).

Meta: these posts that you put up about your travails remind me a bit of the NYTimes column where they present you a medical case mystery. Readers write in with their interpretation, and then the result is finally unveiled.