Ok here a little Explantation:
1.Open GIMP
2.Open a New Image (Background Color White)
3.Filter->Color to Transparency
4.Choose ure Color, whioch us wants to get Transparent
5. Save
BTW you can set a Alpha Value to the Car Window in MED (Skin Settings) Manage the Alpha Value to get it Transparency
cYa Sebastian
Tutorials:
[Blender]Terrain creation ENG/GER
[Blender]Low Poly Tree Modeling
[GIMP]Create a Texture for Terrains
CLICK HERE