I had a shop try to do the very same thing on my 2001 Xterra SE.I had a MTX thunder 4244(or was it 4200) 4 ch amp.The amp had high speaker level inputs and thay spent 4 hr's trying to get the alt whine (motor noise) out.Could not be done.In the end i traded the amp for a new Clarion DXZ-715 head unit.
At 50Wx4 max it's a lot louder and cleaner than the factory 6-disk unit.
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2001 Xterra SE,4x4,V6,auto,Solar Yellow
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2001 Xterra SE,4x4,V6,auto,Solar Yellow