Using the unused roof feature

First I will start off by explaining what this feature is:
Besides the transparent roof shader, this games allows for two entirely different geometry/meshes to be displayed depending on whether or not the roof is entirely visible or semi-transparent.
The game supports an opaque roof shape along with an alpha roof shape. One cannot exist without the other.
With this feature, you can make certain things display only when you're driving the car or when it is parked (your choice!)
Check out this post as example.

How to make it work:
You need to install the following things in your P3D File:

• A shader named {vehicle_name}Roof_m (optionally, with a texture called {vehicle_name}Roof.bmp)
  
• A mesh/old billboard quad group/composite drawable named {vehicle_name}RoofAlphaShape (+ 1 composite drawable prop with the same name)
  
• A mesh/old billboard quad group/composite drawable named {vehicle_name}RoofOpacShape (+ 1 composite drawable prop with the same name)

(Known) Limitations:

• You can only have one of each
  
• The {vehicle_name}Roof_m shader must be present in the file for this to work (it doesn't matter whether or not you use it on any geometry)
  
• Because it waits for the roof to turn back to fully visible, it takes a while for the shape to change back when you exit the vehicle

Thank to the people that helped look into this:

• @Borb (asking questions about it that arose my interest)
  
• @Josh / Proddy (investigated it along)

Enjoy! Note: This post may update as people learn how to use this better