No Sound Rear Speakers Help

[TOLMACH]

Sounds like LPF (low pass filter) is engaged somewhere: either at the head unit or at the aplifier that "cancells" everything above the cross-over point (usually 40-120 Hz)

Check the crossover panel at your amp .. if no cause is found, check HU settings | correct RCA pair at the HU back

Re hi-level speaker cables from the HU to the speaker - why would you connect them if you have amp? No need to disconnect , but make sure the internal amps of te HU are switched off (if your HU allows it, check seetings) to save power and prevent HU "heating"