[JasonACT]

Results:

I hooked up a very large capacitor to each of the voltage regulator outputs, testing each. No change whatsoever. This was sort of expected, there's lots of caps already around that chip. You can see them in the close-up shown above where the brand is clearly displayed.

I connected one of the loose cut amp pins to a random output of the first-in-line audio processor of which there are 4 (via the specified cap) and it remained quieter, but still that little bit of hiss, the same as when unconnected but quite acceptable. Started the SAT NAV demo, but didn't hear anything, so the audio code error state is turning off that chip (it's programmable for each of the 6 inputs it supports - 5 stereo, 1 mono).

Still, that shows the bulk of the noise is being introduced in the processing logic of the device between that main input and the final amp.

Wanting to hear something, I connected that wire/cap directly to one of the SAT NAV audio wires (there's 2: + and - where the audio processor converts those to one output - so I didn't expect much) and I got a constant pitch buzz.. waited a bit.. and in that buzzing a nice clear female voice started reading driving commands every so often at about the volume I would expect.

I wonder what audio processing magic I'm going to lose if I do this to my unit?! Do I... Don't I... Try?