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I ran the wire to a discreet switch near the handbrake and then back to the metal bracket that is screwed to the side of the head unit. You have to make sure that is earthed though - it is on a new-age with a black wire & spade connector.
dragon - just anywhere on the car that is earthed ie, as mentioned in the posts above or somewhere that already has an earth on it like the fuse box or such
You may have a switch underneath your hand brake lever that will have a ground wire so just connect it to that or use the ground wire in the headunit loom.