Ground loop whining sound with aftermarket head unit but stock M490 amp

[Vic S]

Hi guys,

 

Have a 2000 Carrera Cabrio.

 

When I got the car, it came with an aftermarket Alpine that didn't really work well, so never used it much, so not sure if this is an older issue or a new one. Replaced it last weekend with a Blaupunkt Liverpool series head unit - which is a budget unit I have to admit. It uses the stock M490 amplifier to power the speakers. 

 

The sound quality coming out of the speakers is pretty good, but there's definitely some noise (crackling / hum) from the amplifier when the unit powers on and whining sound as engine revs pick up, which is a grounding issue somewhere in the system as it's picking up the alternator. 

 

From my research, it seems like in a stock setup, the grounding for the original CD220 head unit and the amp are linked together - so the harness/CD220 has a grounding connection, not sure where. After all this splicing, etc, I'm not sure if that's been lost somehow. 

 

In my testing, even with volume set at zero, there is the noise and whine, making me assume it's the amp that needs grounding. Firstly, is this the correct assumption? The noise stays the same volume as far as I can tell when I crank up the volume on the head unit. 

 

If I do need to ground the amp and looking at the wiring diagram, I really can't figure out which wire from the blue plug (cause I have a cabrio) that goes into the amp that I should use to create a new grounding connection closer to the amp (I saw a grounding screw just behind the amp). 

[Vic S]

Here are the close ups of the plugs