Sorry if this is a stupid question, but I'm very new to the Prius. Can anyone tell me what this little guy does or is for?
That is the Shift Lock Overide Button which allows you to take the car out of park without a key or during other emergency. If you press it down, you will be able to move the shifter from park without a key or if the electric system does not release normally.
There should be a thing filling in the hole that you can pop out. If yours is missing, I'd ask your dealership why it wasn't in there. It won't hurt anything not being there, but it doesn't look right with it missing IMO.
Mine was in the packet of documentation I got (instruction manual, tire warranty, etc.) in a little plastic bag.
