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My fault, with more testing - the noise starts as soon I turn the key - not the engine...


OK, and the noise doesn't change with revs? Sounds like something [in dash?] that powers up with the ignition and either the signal wires are picking it up or some component shares a ground with whatever it is that comes on...
Not knowing the exact layout/routing of your system/wiring, and being unfamiliar wih Bose systems makes it hard to give exact advise.
For a traditional scheme with HU-RCA-amp I would have said try running a temporary set of RCA wires straight through the interior (after loosening the HU from the dash as far as possible) and running a new [relatively beefy] ground wire to the HU from a good ground spot, possibly, but not neccessarily, the same spot as the amp.
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/Michael