I would try bypassing those amps anyway, just to see what happens.
If all the wiring is correctly hooked up on the HU end, you know the signal is going somewhere and stopping, which is probly at those amps. Maybe the rear speakers are not on the amps, while the front ones are?
As long as there is continuity from the braided cable to the car chassis, it will work.
And just for fun, did you check the fader on the HU? It could be all the way to the rear... I know it's one of those "I'm smarter than that," but it happens, trust me
