A Fat Duck

Hi, Im new here and sorry this is kinda long, but Ive got a problem with my car and thought itd be best solved here. I have a 1997 Isuzu Bighorn(/Trooper) UBS69 3.1DTi with a Fusion headdeck running 4" JVCs off the headdeck in the rear pillar, the three sets of RCA from the headdeck go to a JVC 800w mono block (in back) to power a 12W4-D4 JL Audio sub and the other two RCA go to a Response 4x50w amp (under passenger seat) that powers fusion performance serries 6x9 in the rear doors and JBL 5.25" components in the front. I havent upgraded the ground from the headdeck (told this may be a problem by a friend but after my tests im not sure how this would affect this?) and am running the stock loom into it but I have pretty good grounds on both the other amps.. however I am getting a hissing noise in all speakers (and even the sub). I only just upgraded from a 2CH presicion power amp (for 6x9 and run the comonents off the headdeck) to the response amp but the hissing was still present before this swap so Im sure it isnt the amp, although amping the signal seems to have made it more pronounced through the tweeters especially. The hiss doesnt seem to change with the colume so even with the vol. set to 0 I get the same hissing

So heres what I've tried and found...
With the ignition off and head deck running there is a hiss on all the speakers (incl. sub). I tried unplugging the RCA from the JVC (sub) amp and the noise was quieter on the sub but still there. Also tried unplugging the RCA from the 4CH but the noise was still completely there in both the front and rear speakers.
After this test of the 4CH I tried to run some speakers wired out of the amp with another set of cables that came form the amp right onto the back seat (less than 1m) away from all power cables (try not have it near power cables incase the interferance was the problem) but the hiss was still the same.
I then thought that while I had a speaker out of the door I could listen carefully when I turned the engine to see what happened, so I did this and heard the hiss and another fluttery kind of sound that increased a lot when I reved the engine.

Also, I am running seperate 8G each to the two amps and the RCA, Audio and Power are all together (I know this shouldnt be done but my two RCA into the 4CH amp are supposidly highly shielded and after testing the amp iteself with an auxilery imput and seperate speakers away from the power it didnt seem to make the difference so I thought id ask here before re-wiring the entire car...)

Anyone got any ideas on what this could be? Im hoping its simple but its completely baffled me as I wired my last car with a similar setup and didnt have any of these problems

Thanks in advance

Dukk

How high are your gain settings? Often if there is hiss it is the gains are too high, amplifying the noise floor of the amp/vehicle.

A Fat Duck

The sub is between half and 3/4 but isnt a big problem, hwoever the 4ch is well under 1/4 as otherwise the hissing gets ridiculously loud, even with no rcas plugged into the amp