How to add decals/stickers to car windows?

Hi everyone, im new here, and i have a question. I want decals/stickers on my car windows, when i apply a texture to my window, (black background with white letters) the window becomes black with those white letters, but i cant see thru. I tried making my texture transparent (without background), it looks good in Zmodeler, but when i try to add the texture to the txd file, it says error. Please help, i would be thankfull if someone could reply and explain how to do this.