A relay will take the key operated on/off function and apply it to your "custom" circuit while keeping the loads separate....your dash cam will be powered by which ever fused 12v line you choose, but the switched "trigger" wire is only used to tell the relay when to open or close.

In terms of X5 application, you can use the purple/white stripe wire under the center console drivers side as the trigger wire, which will tell the relay when the key Is on or off. For the camera circuit, you can do it one of a couple ways (all will require fuses obviously). First is for the lazy people: tap into the 12v accessory line for te outlet in the passengers footwell. They have a heavier gauge wire, and shouldn't have a problem supplying the power needed (just remember this when you go to plug something else in though). It has the benefit of no much wire running, and what is required, is all kept behind the glove box. The second way would be to run your own power wire from one of the distribution mains coming from the battery...IMO this is the preferable route since it leaves the circuit for the 12v accessory socket alone.

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