I have bought an aftermarket head unit and a harness adaptor for the BMW.

I'm not great with cars but thought I'd give this a go myself. The harness has a flying red wire which required connection to power. Originally I tried connecting it leaving the red wire unconnected and as you'd probably guess the head unit didn't turn on.

After some reading, every one suggested connecting the red wire into the fuse board (12V), which I have done. I have wired it into an ignition power slot otherwise the head unit remained permanently on. The head unit boots (after about 20 seconds of loading the Android interface) and I can change settings etc, and turn ignition off. If I then turn ignition on again it loads straight away and remembers my settings. However, if I leave it for a longer duration of time it resets the settings next time I turn ignition on, and I get the Android loading screen again.

I have tried changing the red/yellow wires and from what I can see the wires are correctly wired. I have tested with a multimeter to ensure I have permanent/ignition power as and when it should.

I've read the ground wire could be the issue, but assumed the harness I bought deals with this. Does any one have any ideas?

  • I don't know these particular radios. But every car radio /head unit I've seen has at least three connections: a) 12v power, switched with the ignition, to operate the radio/head unit; b) a ground connection; and c) 12v always-on power, so that the programming you've entered (station selection, etc), will be remembered. From your description, I think you're missing the 12v always-on power. Your settings are remembered, but only for a short time; this can occur if there's a small capacitor in the radio/head unit that provides memory power, but only for a short time. Feb 24, 2019 at 21:39
  • Hi, I did test the harness where a permanent 12V should be and indeed it was being provided, and this harness then plugs into the back of the head unit.
    – Ricky
    Feb 24, 2019 at 22:27
  • Did you leave the permanent 12v wire connected to your voltmeter or test light for the same length of time it took for the head unit to experience memory loss? Feb 24, 2019 at 22:37
  • No, that is a very valid point. I will give it a go knowing the head unit will have lost it's memory and see if it has power. Thanks.
    – Ricky
    Feb 25, 2019 at 10:27
  • @David That was the issue, if you wanted to post that as answer I'll mark it to give you some rep, if not I will add the answer myself. Thanks.
    – Ricky
    Apr 2, 2019 at 9:41

2 Answers 2


In some manner, the 12v always-on power isn't always on.

Connect your voltmeter or test light to the always-on circuit, and leave it connected for the same length of time it takes for the head unit to lose memory. You'll probably find that he 12v always-on power goes away. Then you can search for the fault in the always-on power circuit, and rectify it.


I had the same isue. Than it hit me. BMW utilizes a smart battery unit device on the positive terminal.

It's this device I think contain a relay off sum sort and gets timed out after set interval. I just moved my Oterminal to behind it, and voila... Prob solved

Hope that helps guys

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