There is currently a PTS bug I have noticed where it's impossible to alter the installed utility, defense, or performance slot items on every vehicle in the game. Clicking the slot does not display the modules, and as a result, it's impossible to select new modules for vehicles, or install ones when you have none already selected. Same applies to weapons, however they can be cycled through from the vehicle overview/spawn screen. Attachments can still be added without issues to vehicle weapons. This bug does not apply to infantry, but exists with every vehicle for every faction.