I was thinking to change mine as well, but I believe that there is too many items to install and to buy.
first, you'll need the dash controls, which is probably the easiest thing to install.
Then, for the climate control, you'll need the temperature sensors (I think that there are 2 of them). These sensors mesure the temperature from 2 differents places into the cabin. Usually one on the dash, and one elesewhere.
For the MPG computer, there must be a sensor in the gaz tank. I don't think that this sensor is intalled on every Highlander. I think that it is an option which goes with the automatic climate.
Finally, at least another sensor will be required for the outside temperature. In fact, it is just a digital thermometer. I'm not sure where it is installed under the car....
So for me it does not worth the time. Buy a limited it's much more easier !
:whatwhat: