just edit Alpha channel for windscreen DDS file... what is black is absolutely transparent, what is white is not transparent and on greyscale you set the transparency (dark grey very transparent, light grey little transparent, etc ... )
So select area you wanna change transparency, while selection is active, go to alpha channel and fill that selection with fairly dark gray color ... if you wanna keep letters less transparent, make selection from those letters and in alpha fill it with brighter shade of gray
you don`t need to do that in Window DDS file (how it looks from outside), just windscreen (look from inside)