Just as a follow-up – all it was was a drained battery from keeping it in accessory mode for too long while messing with this DYI project.
That taught me I don't want to have my dash cam drain the battery - so
getting a discharge prevention device is probably a good idea. The highest recommended one, LK-290 is on it's way from an ebay seller. I also got the plastic bit for oddments compartment (to replace my BMW Assist buttons thingy), and I intend to mount the discharge prevention remote into that oddments compartment. I shall post pics once I do.
Oh, and that's what the camera looks like, mounted:
