I only activated the trainer when i was in the race and it worked!
I noticed that it would work all the time when it is used with a auto-saved file, but not with a normal save game.
I only really use it for tyres and instead build on parts so the rest of the trainer I do not know if it crashes the game. (well just press the F1 key will crash the game if used with a non auto-saved game)
Hope this helps