I have just finished my build of your extension and must first of all say thank you, for providing everything necessary!
I had lots of fun building it and gladly got everything to work without any major issues.
What I would suggest is to reinforce / reconstruct some parts that are very brittle and tend to easily break off (i.e spoiler arm, gear lever guidance, potentiometer clips, etc…) if those were all designed a bit thicker, you wouldn´t have to worry about looking at them the wrong way 
Another thing I would suggest is the use of threaded inserts that are melted into the holes with a soldering iron. They aren´t really expensive to pick up but make things a lot easier than fiddeling around with nuts, and slots for them that sometimes are too tight, sometimes too loose and the nuts end up flying around the case when you try to screw down the other part. This goes especially for the auto brake and parking brake panel, where you can hardly access the backside of the nut slots.
And I will also change up the ports on the Arduino, to be able to use three PWM ports for the LED´s, as those are just to bright at full power, even with the resistor!