Re: radio speakers have no sound
That's definitely sound (pun) advice.
I recently replaced the post mounted cassette deck in my Esquire, there were four sets of wires, but I still haven't found the speakers the two other sets go to.
I'm under the impression that a pair are supposed to mount in the top of the dash but that's not where they seemed to be going. I just did the two front to the doors for now and will probably run new wiring to back speakers once I find/install a set.
If you can connect the alternate speakers you're testing with to the leads in the cabinet and to the back of the receiver for the rear speakers that should definitely tell you whether it's the wiring, speakers, or receiver.
If the receiver is fairly modern, check if it has a detachable wiring harness and that it's seated properly.
Also look for corrosion on the blade terminals for the speakers/wiring. And if the wiring harness for the receiver connects into the other wiring with blade or torpedo quick disconnects, check those for corrosion as well.
If the wiring crosses underneath or behind any cabinets (meaning it's not embedded in the walls or anything), make sure it's not worn bare from rubbing on something.