@Dr.Acula,
Welcome! That looks like an awesome project!
+1 on the buck converter. You can probably run all of it from that.
Theres a thread talking about some methods for doing different things for different sections here:
I wonder what kind of signal the original dash dimming control has (if it has them), maybe there’s a way to interface it with the ADC, and you could keep dimming using the original controls.
There’s also a few folks that are using Pixelblaze for turn signals and that sort of thing, it would be really cool to have everything networked up for shows!