Hehe
You kind of answered your own question, Den.
It's all about the alpha channels.
DXT1 does not have alpha channel. DXT5 does have an alpha channel. Now, you can use an alpha channel for whatever you want, effectively it's just some extra information in the image.
In GTL, the alpha channel is used for just transparency, so you only need it for the windows and lights.
In rFactor, they use the alpha channel for the glossy-ness. So when you save your body files, you need to save as DXT5 so that the alpha channel information is saved. Hopefully, the person who made the template will have included an alpha channel in the file.
If you can work out how to edit alpha channels in PS Elements, you can get in there and muck about with it. It will be a greyscale image, and black areas will be completely dull and white areas will be very shiny. Generally, the template will already have the correct amount of glossy-ness in the alpha channel, so you can just leave it as it is.
If you want to get really creative though, you can do stuff like make the decals less shiny than the bodywork, or make dirty areas dull and clean areas shiny.
