Do you also have a problem with your aux in levels being too quiet, but if you turn up the volume on the other head unit then you get distortion?

I didn't have much alternator noise, but I had the aforementioned problem, and I spoke of it to Hugo, and he said "Hm, that's odd, it sounds like the outputs from the other head unit aren't grounded properly."

So, I eventually got around to taking off a bit of the dash, and there was a "speaker to line level" converter in between the other head unit and the empeg. I tried putting the RCA grounding leads that came with my Mk1 in there and it worked like a charm.

What happens if you actually properly ground those RCAs?

Richard.