I read about that, but it seems that the internal disc drive transmits digital data all the way until the main processor (or whatever connects to the amp input).

My bad - yes I mean an AUX input I thought I would've needed to ground the unused lines (not sure if I should connect to ground directly or with a resistor) - you think that could be the cause of the whine? The whine comes from the front speakers too. And I really can't understand how and why I can still get some signal from the stereo...

It's usually a good idea to look at what's normally connected, when breaking a circuit, and replicate that. Which I seem to remember is a 10k resistor with a parallel capacitor (being too lazy to go back and look again).

There's a resistor, then parallel capacitor and resistor to ground, and finally a capacitor to the amp. I could try replicating the parallel pair if that could help

add a series capacitor, if you can.

Would that be a polarised capacitor for protection? And on both the original and my added lines?

The way I modified the circuit, the AUX goes into the .2μF capacitors that was already there before the amp input lines, is that sufficient?