Interesting. Mine has it, but I had no idea it was a simple switch if you had a diff lock.If you have the locker, but not the ATRAC then yes, you can just add the switch as everything else is in the "computer " and ready to go. When I bought my FJ it was a pretty basic model and I had read about the switch and bought one. A couple of days later after I bought it it took me less than a minute to put it in and I then had ATRAC. A lot cheaper than paying the $2500 for the package it was a part of.
There are also some other switches that are pre-wired too like the wiring for roof lights and fog lights if I remember right. You add lights and a switch and you're good to go.