ok, the reason i say use one ground is like i said, using different ground points will increase resistance between the different units (say stereo is grounded within 6 to 10 inches of stereo, but amp in trunk is grounded 6 inches from amp - so you're looking at about 7 feet of steel between the two. if either one of those grounds loses even the tinest bit of its integrety, you're looking at massive amounts of noise that will be created. if all units share the same ground point, chances of that happening are reduced by a large factor. anyhow, when it's not practical, you don't HAVE to do it that way. i've got a couple of questions... do you have any electronic crossovers, or equalizers, or anything between your head unit & amp? if so, when you unplug the head units rca's from the crossover/equalizer, does the noise go completely away like it does when you unplug the rca's from the amp(s)? it should be clear that the noise is obviously coming from before the amp, and i can tell you why. amplifiers get their power from an isolated source - they use a switching power supply similar to an AC inverter, that converts 12vdc into AC, which in turn is rectified & regulated & filtered somewhere between 36 & 80 volts DC (combined - almost all amps use a center tapped dc output, the tap being the divider between the left & right channels). this is why amps that have such high sensitivity & power can have no noise like you said, when unplugged from the stereo. unfortunately, until recently, almost nobody has made head units or any other support gear that uses switching power supplies to isolate their power too. apparently your stereos low level output is such a low voltage that the amount of noise getting past the charging system that isn't filtered out, is being amplified as a differential voltage. you can try putting capacitors on this, but the thing is, the waveform that's getting past the battery that's causing this noise is so small, that it would take probably a 2farad or more capacitor to cut it down by a noticeable amount..... but it will be an exponential increment you'll need to increase the cap size for each factor you're trying to filter out. there have been two extreme cases i had in very exotic systems i've done where i completely removed 100% of the motor noise - but it takes an unusual solution to do it. first thing i did was get an AC inverter & hooked it up to a relay thats switch to turn on was from the key, but the relays output was tapped straight from the battery. then i plugged in an AT computer power supply to the inverter, since it uses a very stable filtered switching power supply, that has enough 12v+ current output to run 10 stereos. mind you, the computer power supply runs everything EXCEPT the amp. it powers stereo (cd or tape player, or just head unit), disc changers, displays, electronic crossovers, spectrum analyzers, any of the audio equipment that you are running except for the amp, which gets its power straight from the battery, since it has its own switching power supply & doesn't need to be filtered. you must connect the computer power supplys ground output (the black wires) to the cars chassis for this setup to work, and don't use any of the other output voltages from said power supply unless you're using for some other project. this setup may seem strange, but with it, you can start the car, & remove the battery and you will hear NO motor noise in the stereo (just so long as you keep the motor running at enough RPMs to keep voltage high enough to keep the car running while powering stereo that is). extreme needs can inspire exotic answers ..... if you need help with making something like this, leave another note & i'll tell you how to get hold of me