this is what i bet the issue is



you have a fancy head unit that has filter built in for high pass and low pass.

the tech probably set a high pass filter on the head unit to limit the bass to the front speakers. the car only accepts input from front input on your head unit and then splits to rear at the amp so all signal modified for front affects rear as well.

most members put capacitors at the door it self to limit the bass.

now the frequency is as follows and this is a rough each user picks his settings

subs: 60hz-80hz and below

mid bass: 63hz-300hz

mid range: 200hz-3k

tweeter: 3k-12k

what they did was set a filter and block all frequency below a certain range so limit 80 hz and below and that is where the main bass notes are at between 30hz-80hz.

if you disable the filters on your headunit you should be able to hear the bass on your speakers in the rear

again keep in mind the speakers in the rear which have a port hole is subwoofers no voices will come out of it but low end notes.

cliff notes: disable all filters on the head unit since front wire on head unit is split to all speakers in the car.