Vanilla, those cables are not 100%, I have gone through two - the second having the same problem as you described, the first not at all. A bit disappointing, but at least they make everything else really easy.
Grab a 12v-6v voltage dropper and hard wire it in, it's what I did and it works great now. Bonus being that I can then change the headset to reverse camera display at anytime.
This is the one I used; i'm sure you can find something similiar in Dublin
https://www.jaycar.com.au/dc-to-dc-s...odule/p/AA0237