liamstears In case this hasn't been solved in the meantime: I encountered the same alternator whine when I added an amp last week. I was a bit lazy and connected the amp to the same harness that the Dudu7 radio lives on, after all it's a small and rather efficient Class D Amp (Eton PA 2), connected via RCA. Ended up with bad alternator whine that I didn't have before. So in case you or someone else needs this:
Issue: Alternator whine. Ground loop.
Cause: While VW (and I assume Volvo as well) has rather beefy cables on the car's side of the radio connector (Power Quadlock), Dudu's (Skyfame's?) quadlock harness that was supplied with the radio (VW kit) has only thin wires for 12V and Ground. They seem thicker than the others but mostly consist of insulation and only thin copper wire. That's a bottle neck. The amp consumes more energy than the radio and wants to route more electrons back to ground than can be squeezed through the thin ground wire of the harness. As a result, the amp routes some "excess" electrons that don't make it through the ground wire easily via the RCA cable to the radio instead. The shielding of the RCA cable no longer works, alternator whine can be heard.
Solution: Route a new and sufficiently beefy (6mm² in my case are good enough) ground wire from a known grounding point of the car (workshop manual lists them, use the same point for radio and amp) to the radio and amp installation places. Cut ground wire in radio harness and connect radio ground to new ground cable. Connect amp ground to same ground cable / grounding point. Noise gone.
Alternative solution: You can think about just replacing the harness ground cable with a beefier cable, if the car side has proper wire. I didn't go that route because a new ground wire was easier and safer.
NB the 12V in the harness is too thin for any amp also, even if the car has proper wires. Again use a proper cable. But 12V only influences the power of the amp (voltage can briefly break down when the amp uses power for a bass etc.), not alternator whine. I have my amp on a separate 12V cable also.
