2007 altima ground noise with stereo off

I have a question about the dash speakers in a NON Bose 2.5S (2007 Nissan Altima 2.5S).  I installed everything in my car about a month ago.  I have 3 amps, 2 12's, 2 6.5's, and 2 6x9's.  The only speakers I did not touch were the ones in the dash.  I piggybacked all of my mid-range speakers from behind the deck to the 4 channel amp (6 speakers including dash speakers).  I am now getting clipping on the 4 channel at high volume.  The problem with this is that it is only happening at certain times.  I start the car and get a humming in the dash speakers WITHOUT the radio powered on.  Does anyone know what is causing this?  Also, the 4 channel ONLY clips when the speakers make the noise.  If I start the car and that noise is not there then everything plays just fine.  I have come to learn this is a VERY finicky car.  I really don't like that about it.  Any suggestions on this?  Should I disconnect the 3 inch dash speakers all together?  Are those speakers (3 inch factory dash speakers) changing the ohm load somehow?   ALSO, we have already tried to figure out how to disconnect those speakers and cannot find how or where to get to the backside of them.  Anyone that can tell me how to disconnect them?

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