You knew this was coming sooner or later. Android Auto has been viewed as a mostly closed system, if not by nature of the software itself (which is technically open source) then by the fact that it's an extension of Android running on a big chunk of metal installed in your car. But some intrepid developers over on enthusiast forum AVIC411 have discovered a way to boot software directly from the SD card slot on Pioneer's NEX series of aftermarket Android Auto head units. That paves the way for custom ROMs in your car, or at least your car's entertainment system.
The process for booting up a custom image is long and involved. While you don't need any particular technical skill to follow the 9-step process presented by forum member bass_rock, you will need quite a bit of patience. Users need to reboot the head unit into test mode, then install a customized version of the BSP software. The creators explicitly state that performing the steps incorrectly might brick the device, so it isn't for the faint of heart.
At the moment there aren't actually any custom ROMs available - no one has gotten around to making them yet, since the SD card option was only published on Monday. The creators of this mod are hoping to get ADB, a Network Bluetooth Profile, and USB OTG working, with the possible bonus of porting Android Auto itself to older, similar Pioneer head units.
- Android Central