"Well I'll tell you something. This is no longer a vacation! It's a quest. It's a quest for fun! I'm gonna have fun and you're gonna have fun" - Chevy Chase (seems to fit my quest for higher gas mileage)
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From what I've heard, the TCH doesn't come with the capability to provide that function. The mod for the Prius is because they're made so in some markets they come with the EV button. And last I knew the Highlander Hybrid came with an EV mode button (well that was what I read before it came out anyway). But far as I know we're out of luck. Just have to learn to master "full hybrid" mode, to try to maximize your EV availability. But there's no way to run in EV before the car's warm, for example. Sorry...
Steve (chestr) is correct. What makes the Prius EV mode operate is the computer's (ECU) programming along with a momentary switch (not available in US/Canadian units) to enable the function. Unfortunately, that programing function was not written for the '07 TCH, so just adding a switch won't give us EV capability.
John
2007 TCH -- Desert Sand, Bisque -- Original Equip: Leather Seats; NAV; Moonroof; Heated Seats
Completed Mods: Moved the side "HYBRID" emblem/badge to trunk lid; Mud Guards (Colored); Back-up Sensors; Door sill protector; ScanGauge II; Front Bra (Clear LLumar); Window Tinting (40 % Charcoal LLumar); Rear Deck Lid Spoiler
Mods in progress: None
Future mods: Rear bumper appliqué, Sliding Center Armrest
EV Button for TCH?
Unfortunately, that programing function was not written for the '07 TCH, so just adding a switch won't give us EV capability. 
