Is there any way to write on the window of a car? I wanted to add some text on the windows of like a K-9 car, but wasn't sure how to do it, or if its even possible. 

Some cars have windows templates properly mapped, while others use the default shared GTA IV windows texture, and some just use a really small transparent texture. For cars with the last two, you cannot, however if there is a windows template available for that car then yes you can.

Thanks for the help!