To find it, you'll likely need to remove or partially remove the lower portion of the passenger side dash panel to access the area behind it. It's usually a small, easily identifiable square or rectangular box with a button.
If you can't find it in that general area, referring to a repair manual specific to your 2006 Ford Ranger would be the best way to pinpoint its exact location. You can often find these manuals online or at auto parts stores.