The wiki exists for a reason. https://wiki.beamng.com/index.php?title=BeamNG:Installing_Content&redirect=no
Hopefully you have the folder downloaded, drag it to your desktop. Then open up file explorer, then go to Documents, then BeamNG.drive, then mods, drag it into there, launch the game, and look for "Ford Mustang"
It's a bug. Turns out when I renamed the transmissions I forgot it rename them in the config files. Now the GT configs have no transmission lol
can you keep. the no damaged windows they are the best just do a little node grabing and the windows are "DowN"