Just to follow up, I have spent the whole day in the garage, trying to fix the issue, in a word, a nogo, very frustrating since the mk2 is unusuable in the moment. Here is what my installation is like and what I discovered.

Front: Single 2-ch. amp directly connected to the mk2 front out.
Rear: A active crossover connected to the mk2 rear out, to the crossover there are 3 amps attached, sub/bass, middle, high.

First, I disassembled my gfriends car again :-( to get the grounding wires back which came with the mk1 which worked absolutely FLAWLESS just to mention it again.

Put the grounding wires in - no change, it even got worse. Now I got a hum added to all that noise, so took them back out.

Started working on the rear amp's. I took out the active crossover and switched on the amp's built-in crossover instead. Bang. Away goes all noise, hum, whatever. Crystalclear sound. Put the xover back in, back is the noise. Checked grounding of the xover, grounding is perfect. The I just connected one channel coming from the mk2 to the xover, and something funny happend - I discovered with just 1 channel connected, the noise comes out of the left and right speakers! Tried the other channel - the same. When I connect both channels, the noise get a bit louder. So to me it looks like the MK2 throws RF via the wire into the xover which might be a bit picky on that.

Then I tried to put additional grounding to the wires, I connected the shieldings of the cables coming fom the mk2 to the car's chassis. No change, but a additional hum was added to the noise. The same effect as there was when I put the grounding-leads at the mk2 (which do nothing else than putting the shilds to the car's chassis, so the same I tried now).

For now I left the xover out, but after all, this sucks big time and I don't know what to do next.

Went to work on the front system. There is just the amp connected straight to the MK2 with maybe 30cm adio-cables. No matter what I was doing, I couldn't get the noise out. Checked all the grounding, all looks perfect.

What is frustrating is that I haven't changed anything in my installations in the first place, compared to the MK1. I really expected it to work without any glitch, and now that.

If someone out there has an idea, I would really appreciate that. Or I have to buy new gear which I don't want. Thanks, J.