On my Polo (´00) it's actually a pulsewidth modulated signal (IIRC ~85Hz) and wasn't suitable to drive a relay directly. If it's the same in your Golf you'd probably need a way to integrate the signal into something much slower moving, then maybe a comparator (preferably with some hysteres) to get distinct 0 and 12V levels. I'd look at some other wire if at all possible for what you want it to do.