Non transparent car glasses

[Arno10]

Hi guys,
I'm not English but French so sentences won't be perfect.
As a beginner, i need some little help please.
I try to modify sponsor on rear and front car glass but after changing the main sponsor and save file in DXT.5 or DXT.3 or every other, every glasses turned black, i think it's about alpha channel, maybe, but i'm not familiar with that si i don't know how to rectify.

Before:

After:

If someone have a few minutes to help me.
Thank you guys.

 

[Hoksu]

In alpha channel, white is opaque/painted while black is trasparent. Not familiar with that particular car but generally banner, decals should be white in alpha while the rest should be dark grey. Black makes it look like there is no glass.

Example alpha:

Result:

Export it in DXT5 format. And make sure the alpha gets exported or it doesn't work properly. With my ancient Photohop I need to flatten the image before exporting or alpha doesn't get exported

 

[Arno10]

Hi Hoksu,
Thank you for your quick answer and thank you very much for the solution because it worked with flatten the image, exported in DXT5 format.