Probably the easiest upgrade for your Toyota FJ Cruiser:
As you may know, FJ Cruisers equipped with a Rear Locking Differential, but NOT the A-Trac button can add A-Trac functions very easily:
Second, take a small screwdriver (I used the smallest head on my Leatherman) and push it on the top of the BLANK right next to the RR Diff Lock button. Pry up slowly and the button will pop out.
Unplug the connector from the blank. Plug the connector into your A-Trac button. Push the A-Trac button into the hole until you hear it click in.
You now have A-Trac. Congratulations!