BA Door Speakers not working

Hi guys,

My BA has a JVC KD-G825 in it, noticed the other day that the door speakers aren't actually working. Only the rear speakers up near the back window.

I tried playing with the fade settings to adjust the balance to the front speakers but nothing came out of them so I assume they are not hooked up to the JVC unit. The car did have an aftermarket amp & subwoofer installed when I bought it so I guess things have been messed around a bit.

Are there any guides to show how to hook up the door speakers to an aftermarket system? I've got no experience in setting up such a thing so looking for a place to start diagnosing the issue.

Thanks.