It could be a number of things. I could be coming from your deck and a simple noise filter could solve this problem. It could be where you have you amp grounded to or the ground line could be to long, its best to have it 3 feet or shorter.
I'd really like to know why so many people say that. My ground cable from the actual grounding point to the distribution block it feeds would be around 5 feet long. Then you have to add the additional length of cable coming out of the output side of the distro block going to the amps. I have zero inducted noise.
If you have grounded and re-grounded and re-grounded to no avail, Ben, then I would suggest your problem lies elsewhere.
What kind of head unit do you have? If a recent year-model Pioneer it is possible that you're experiencing the infamous Pioneer RCA output ground trace problem. Apparently if you connect or disconnect the RCAs while the HU is powered on it burns something on the board internally and creates an inescapable ground loop problem that can only be remedied by sending the unit in for repair.
If that isn't it then it could possibly be bad shielding or faulty ends on poor-quality RCA interconnects.
It could be your speaker wire got pinched somewhere and has compromised the insulation jacketing and allowing in inducted noise.
It could be bad spark plug wire insulation creating RF energy to escape into other cabling.
There is no one cut-n-dried solution to the problem - you've got to hunt through your system and play Process Of Elimination until you've located the problem then correct it.
I wish you all the luck in the world, fella - this is often times a mentally painful and time-consuming process to track down the culprit. Rest assured you'll not be the first soldier to fight this particular battle.