My 04 has the factory HK system with an aftermarket head unit installed with the harness adapter that uses the RCA outputs of the head unit to the factory amp under drivers seat.
Front and rear speakers sound great, no problems there but the subwoofers in the rear door are super faint despite the volume level.
I have a feeling the previous owner did something to them but not positive about that.
I don't see a separate amp in the rear door for those subwoofers (unless I'm missing something) and from combing over wiring diagrams I don't think there is a separate one, looks like they are powered from the factory amp under the seat.

Any ideas on what's going on? System sounds great otherwise but I'd like to have the subs in the rear door working.

Thanks in advance, I've searched and read all kinds of audio threads.

 

Subwoofers are usually faint and only produce deep bass. imo

 

I have the same problem in my p38 someone installed a aftermarket head unit, in my disco 2 I installed the alpine head unit and the sub woofer hits good.