Has anyone tried to fit a bluetooth phone kit which assumes a mute wire is available from the back of the head unit? I have a period Becker 754 which has no such wire and i have read that the way to get round it is to put a relay into the accessory power feed so that when the handsfree kit tries to mute it cuts the power to the head unit. However, what relay should i use and how should it be wired up?