The Japan and European versions of the Toyota Prius are sold with an EV Button pre-installed by Toyota. For some reason, in the North American version this button is left off (note: speculation focuses on possible EPA/NHTSB regulations that were unclear as to whether the option to use 2 different 'fuels' was legal). Also it has been speculated that the decision came from Toyota, but no clear statement, at this writing, has come from Toyota.

However, the ECU is programmed for the EV button. Accessing this feature is in theory a simple procedure. One simply needs to plug a wire into the proper plug on the ECU, attach that wire and a ground wire to a momentary switch and EV mode is available.

Unfortunately, accessing the proper pin to plug the wire into the ECU plug, and threading the wire and ground are a bit tricky. That is where a company named 'coastal technologies' comes in, they make an easy to install kit that allows anyone to add the the 'EV mode' functionality to their Prius. Although there are many different ways to add the 'EV mode' capability, I think the Coastal Tech solution is by far the easiest (not the best but the easiest). However if you are a little more mechanically inclined like myself, then you may prefer the factory style mod I did on our 2006 Prius which uses the actual Toyota EV button from Japan.

Link to Coastal Technologies website: Coastal Technologies