Actually, I noticed this too when putting my beetle back together (after fitting the tuner - my dash had been in pieces for months as I tested different things). Moving the RCA leads nearer the fan unit (and away from a large bundle of cables in the dash) got rid of the noise that I'd managed to introduce previously by stuffing the cables deep into the dash.

It's all nice & neat now, amazing how neat it is when your cupholders aren't full of torx screws from the dashboard :)

Hugo