[thinking] But then why would the noise be there also at home (therefore using the two rca connectors)? Mmmmmm.... [/thinking]

As to the ground loops in the car, I am quite sure it is not that problem since the ground noise has just appeared in both my and his car (completely different installs), and at home, and only occurs with his empeg and not mine, so this makes me think it's something inside his empeg...