Hi, i have a annoying issue with a Sony Dab HU Alpine amp (previously had a pioneer Dab and pioneer amp with same issue) ..If i connect just the front speakers to Channel Aon the amp sound is fine with no noise ,gains set via multimetre to correct volts for my amp..Problem starts when i connect the rear RCA  channel B to the system(no rear speakers connected when doing this) that is when i get the humming also when i use a indicator the peaker picks up the signal also when i put my foot on the brake it buzzes through the speaker ...I don't have the alternator whine just the annoying buzz on rickover and the brake and signal noise..RCA cables changed to eleminate them ,also used a few different ground locations and everything checked , i have placed a ground loop isolator on Channel B and that reduces the noise but that also degrades the quality ,i suppose if the Ground loop isolator improves the issue then would one say it's a grounding issue and not electrical even though i get noise when i brake and use the indicators ..I have ordered a power noise filter that goes in line with 12V constant and ignition in hope that helps ...what gets me is no issue with fronts but as soon as rears are attached boom the noise arrives ...Any help or advice would be apreciated .Thanks

You got a bad earth some where. 
get an earth grounding kit & wire it up.

i had this issue a few years back on my Evo.

got an aftermarket ground wiring kit & it sorted out about 95% of the issue.

Hi, i have tried everything even upgraded to 4 gauge both power and earth cables..Looks like it could be worth a try

Yea are your plus & negative connected the right way round on the rear speakers some times a little mistake can cause big issues