I purchased the IR 18 Indycar and it shows as hidden when I try to access it in my inventory. I also tried looking at the list in rFactor 2 properties and could not find the car. Any help would be appreciated, thanks!
The hidden thing in the inventory is something steam has changed, but it should still install correctly, it should be under Indycar in the car list. Make sure you have All Cars And Tracks selected.
That is the current state for all or most of the DLC content. I don't know why but all you can do is view the car's icon at your inventory page. The car will, however, show up in game. Look under Indycar at the car selection screen. Should be version 1.43.