[Tim Weller]

I don't know how to create an Alpha Channel in GIMP, but you need an Alpha Channel to make a .dds transparent. If that channel doesn't exist, exporting as DXT5 won't matter as the format will still need that channel.

Here's a google result that probably helps? : https://docs.gimp.org/2.10/en/gimp-layer-alpha-add.html

A better question might be... which car are you doing? Not all cars have a mesh for the entire window... most of them just have a tiled tiny 16x16 texture that is transparent to cover all the glass. In those cases you won't be putting your name/number on those windows. The ones that DO have numbers and such typically have a separate texture for it. If you use Content Manager, use the CW Showroom to click on another finished livery's window text and get the name of the texture used for it then edit that specifically.

Hopefully something above helps.

Thanks for your reply! Much appreciated. I have kind of bodged it for now; not really had the time to investigate properly. 

It was for the windows of the Mini JCW, which typically is where the name and number are placed...