Here's the post I made about my EV button. I can't seem to find the guide I used, but here's the jist of it. I stole an unused pin from a connector in the steering column and added a long wire to it. Unplugged the ECU connector and added the pin to the correct spot. Routed that wire through the dash to the far left side. Added a switch next to the dimmer, wired the switch to the wire from the ECU and the other side to ground. Not too bad, especially if you've ever done any car wiring.
This page describes the requirements and limitations of EV mode.