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Switch Feed to Aftermarket headunit

Kev_M5KPL Dec 8, 2008

  1. Kev_M5KPL

    Kev_M5KPL Member

    Hey guys i have been reading on a few forums how to get a switch feed to my aftermarket headunit as it will not turn off when keys are out of ignition.

    i have read you need to run a wire in from somewhere under the dash to the headunit.??..

    Can anyone help on which cable i need to connect the switch wire to as am totally lost at the moment?.


  2. AndyMac

    AndyMac Moderator Staff Member Moderator

    Not sure on the 8P A3 bu on the B6/B7 A4's it's the ultra skinny red wire coming from the ignition. It's the only one among a whole bunch of 5 amp cables so it's really easy to spot as it's very very thin. Because it's so thin you can't run the HU directly off it. You need to cut it, then rejoin it using a small terminal block whilst adding an extra wire. Run this wire up to the HU then use it to trigger a small relay. Wire up the relay using the perm live & earth already there, with the new wire as the switch on trigger and then the remaining terminal going to the "ignition on" feed on the new HU. This will give you "ignition on" turning the HU on and it will stay on then until you pull the keys out.
    The easiest way to ensure it's the right cable is to use a 12v testing screwdriver with a sharp sewing needle attached to it. Then use the needle to poke through the insulation to check it's live with ignition on and stays live until you pull the keys out. This way you're not damaging any of the wiring unnecessarily.
  3. tommytoop

    tommytoop New Member

    You the man Andy, just finished installing my Kenwood HU in my A4, just as you suggest, feeding a relay of the small red key switched wire. Not that easy getting the wire from the HU through to the barrel mind you! these things are sent to test us!!

    On my 2005 A4 original Non Bose Concert Half Amplified rear system I had to fit the relay switching the permanant car loom feed to the HU (red iso wire) and also to the rear amplifier feed through a 5A fuse. The permanent feed then connected to the stereo memory (yellow iso).
    Had to put the amp feed through the relay so as not to flatten the battery!!
    Soldered on some rca cable sockets onto the factory amp channels and plugged my HU rear(sub) rca's into them.
    Happy days, sounds pumping. Need a fair bit of fader to the rear as the back's a bit quieter, I can live with that!!

    Now all I need is to butcher a 99p ebay 12v ciggie usb supply to fit in a small box behind the HU, supplying extra power to my Hard Drive as the HU won't.
    Loads of music to drown out the wife and kids.

  4. AndyMac

    AndyMac Moderator Staff Member Moderator

    Nice one. You could have just connected the amp feed to the remote on from the HU!
    Also you really need to rewire the rears directly to the HU then use the sub out purely for the sub, that will bring the rears up to match the fronts, but you're nearly there.

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