Make sure to use a strain relief on the cables as they pass through the holes you'll need to drill in the rear plate. DON'T DRILL THE HOLE UNDER THE USB CONNECTOR CUTOUT AS SHOWN HERE! Move it down a bit, as the USB connector is typically a right angle connector, and may interfere. It was necessary to trim the connector housing to make it fit.

Add some hot glue to the edge of the PCB to stabilize it; the car dashboard is a horrible place for electronics! Make sure that no points on the back of the PCB can short to any of the case parts.

Testing the completed mod before you put the whole unit back together is an excellent idea. It is possible to apply 12V to the unit on the bench: first place the upper PCB in place, and then temporarily reinstall the front panel. Apply 12V from a bench supply to the main input connector (refer to the schematic and layout drawings for more info).

Reassemble, reinstall, and enjoy your hard work!