It's adjusted in the database.
Find the driver number you want ie: Michael Schumacher is driver 40.
Then click on the Helmets tab, and right at the right hand side at the end, you'll find the fin.
1=full on
0.5=semi transparent
0=off
Enter 1 into this section to change Schumacher to full fin....for example.
It works, but just for the grid drivers. Still don´t work on the helmet of my carrer driver