Aristo - Audio Issues

[durfsurn]

Hi,

I have a few issues with my sound system in my Aristo - fitted with the JBL set up.

Firstly my rear left door speaker was crackling and popping so I replaced it with an aftermarket solution - however that didn't fix the problem. The front centre speaker also crackles lightly under certain audio conditions but I'm unsure if its the same issue.
Separately I also have an issue with the subwoofer cutting in/out on its own accord.

I'm now guessing these two issues are related, is it more likely a wiring issue/grounding issue or the amp unit itself? How on earth do I go about debugging this? Is there a wiring diagram that shows which wires go in/out of the amp? At least then I could try and piggyback off the signal to eliminate the wiring between the stereo-amp and/or amp-speaker.
 

Any thoughts or past experiences with this?