We bought our 2012 Enclave 5 years ago. At the time we bought it is already had a rear game/video unit installed in the ceiling. Several years ago the battery went dead - we weren’t ever sure why – now it’s had two batteries fail within a few weeks and the only thing they can come up with is that the unit installed is “after market” and it could be draining the battery??? Why would it just start to do that after 5 years?
Because it is installed inside a car. And cars get very hot inside in the summer and very cold in the winter. Those temperature differences make electronics fail. They don’t fail on products made by the auto makers because they use better quality, more expensive electronic parts.
Mechanics can test for the power drain, and hopefully they have and confirmed it is the video unit. We would have guessed that was the problem even if you had omitted the mechanic’s assessment.
Have the units disconnected and see if that solves the problem.