I am not sure of this goes here or in the meshing subforums...But I was wondering if it would be an easy task to make vehicules in the game, or just some of them, with opened windows..instead of closed.
Not only it would look good, but it would be easier to put mouse on the driver if you'd like to know his/her name...
But mostly for the look.
Would it be a metter of texture? or editing the window mesh?

Almost certainly you'd have to alter the mesh, the material definition for most vehicle windows don't even use a texture that you could edit.

Great!
I can do that...
Thank you.

Most cars don't have the internal windows textured which, once you have a clear view of the inside looks odd particularly the windscreen so you probably should fix that while you're editing the mesh.
If you have Freetime or higher installed you can add opening/closing windows to vehicles as an option.
An example of The Yomoshoto Evasion with open windows as an option can be found here:
https://app.box.com/s/9ugr3chblzmopwc9wtveuc1hgb4whwsi
Freetime added the paint option to the car pie menu which runs the code on the car instead of from the car globals so we can abuse that to add options that are specific to each car.