We installed a Thinkware Q800 Pro 2-channel dash cam and Blackvue B-124 battery pack in this brand new BMW X5. As with previous generations of BMW, a battery pack is highly recommended if you want parking mode recording. This is because the car's computer tends to throw error codes if it senses a battery drain that it did not expect while parked, in this case a hardwired dash cam. The battery pack simply charges up while driving and becomes the sole source of power for the dash cam when parked.
We were able to get power for the battery in the rear fuse box and store it in that compartment as well. The Thinkware hardwire kit needs to be extended to reach the rear as it's only 10 feet long but this location offers a cleaner install with the battery.