just installed two 'dunno how its called in english, it electrically separates the head unit from the amp but lets audio through' just before the amp, now its crystal clean i the car too

You're referring to "Ground Loop Isolators", and they can be purchased at Radio Shack here in the states.

Usually, if you're in a position to need those ground loop isolators, you've got a grounding problem in your amplification system and it could be solved more cheaply just by reworking your grounds properly. But those isolators sure are easy to install, and they do the trick for a lot of people.

Good tip.

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Tony Fabris