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I have an aftermarket head unit in my 2014 Jetta that I installed myself. After several times waking up to a dead battery when I was sure I didn't leave the radio on, I decided to kill all power to it when the ignition is off.

There are, of course, two power wires coming from the wiring harness. The yellow one is labelled "12v battery / constant", and the red one is labelled "12v ignition / switched". The head unit also has two wires for power, but I wired them both to the red wire anyway to make sure it didn't get any power with the car off.

After another dead battery and then verifying that the radio does indeed continue playing with the car off, I took a volt meter to it and confirmed: the yellow and red wires each give 12.5 volts regardless of if the car is on or off. How can I resolve this?

  • Welcome to Motor Vehicle Maintenance & Repair! – Pᴀᴜʟsᴛᴇʀ2 Aug 15 at 16:23
  • "but I wired them both to the red wire " this was your mistake. – Moab Aug 15 at 17:18
  • @Moab Why? The only downside I see to that is lost settings, but I don't care about that, and it doesn't even do that anyway because the red wire has constant power when it isn't supposed to. – zondo Aug 15 at 17:37
  • If the red wire labeled "12v ignition / switched" provides power even when the ignition switch is off, then the red wire has been misconnected by someone so that it provides power all the time. It should not do this. You'll have to trace the red wire back forward to find out what's been done to it. – DavidSupportsMonica Aug 15 at 17:52
  • I think you should disconnect the unit completely and then test the wiring loom supply lines. It is unclear from the descriptions which wires you have shorted. I wired them both to the red wire anyway to make sure it didn't get any power with the car off. Wire it according to the manufacturer's spec. – Weather Vane Aug 15 at 20:20
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It sounds like your wiring harness (the car side) is miswired, i.e. your switched power is actually constant power. You need to fix this so that the switched power terminal actually uses switched power.

Easiest thing to check is to remove the head unit (and as much of the surrounding plastics as practical) and peer into the wiring on the car side. From the factory you should have wires going into OEM terminals. A well-done head unit install uses an adapter harness that looks like this:

enter image description here

If you have wires changing colors, random electrical tape, homemade-looking connections that's your cue that someone was in there and hacked the wiring.

Next, look online for a wiring diagram for your vehicle for the head unit. These are generally widely available. Verify that the wires you are looking at in your vehicle match the colors that the wiring diagram says they are supposed to be.

If you go far enough into the harness you SHOULD find the proper OEM wire colors and proper functions.

If you can't figure this out, a hacky way out of the situation is to find some other switched power source. Most things on the car use switched power so this shouldn't be too difficult. Problems with this approach:

  • Your radio will be on some random fuse.
  • You could overload that fuse.
  • You could have electrical interference from whatever else is on the circuit you'd be using.
  • The fuse may be rated too high and your radio may burn out in a short.
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