Anyone know what this plug below the steering wheel is for?

It's not the rear window or traction control, those are plugged in.

 

It's for an option your Limited doesn't have.

It reads here in the Toyota 2008 Tundra Quick Reference Guide, it is for a 115V AC Power outlet ON/OFF switch.
Pages 2 and 3, Switch 22.

2008 Toyota Tundra Manuals & Warranties | Toyota Owners




My 2014 SR5 has 3 or 4 blank spaces in the dash that have plugs behind them.

Hopefully, someone that has one can chime in to confirm.

 

Also dis https://www.tundratalk.net/threads/what-do-these-do.104365/

 

These trucks are pre-wired and have quite a few plugs that are not used depending on trim level.

 

Well that explains a lot thanks fellas.