As you've probably guessed by now, adding see through glass to the engine has opened up a whole new can of worms. First was getting interiors in each car with the minimal work possible, second was actually getting it looking good from both inside and outside.
Rather than go down the route of drawing each glass texture by hand for each vehicle, I extended the decals system to allow users to simply draw on the glass just like they can already do with the decals on the car paint. The only difference is that I needed to add alpha to the textures and add the ability to change the alpha for each decal that I place on the glass. This is so users can put sun strips, sponsors, and anything else they want to on the glass.
Once this system was extended it was very trivial to then knock up some glass detail for all of the vehicles in the game, and then leave this system in place for other users to create their own glass textures with.