I love the idea and it’s something I thought about doing too, so I’m super interested in this thread.

For me, I thought it would be interesting to remove the existing stereo and mount everything it the double-DIN hole left behind. That would give you access to power, ignition, and speaker wires easily. You would need to do some rewiring between cars, but it would be the most practical spot.

From working on some fleet vehicles before, I’ve seen interesting systems where all of the accessories were on their own battery that only charged when the car was running or off an external charging cord that could be connected if it was going to sit for a while.

The benefit of a separate power system was that the equipment didn’t need to boot up every time you started the car. Maybe the display and anything else could be powered only when running, but if the main computer was always powered that would save time. It could also get a signal from a battery maintainer that the accessory battery is low and perform a graceful shutdown.