Just in case anyone is thinking of fitting an auto dimming mirror I wanted to share my (positive) experience and add some pointers.
I bought a Gentek 50-GENK41A off ebay for $169 inc S&H which provides the auto dimming and the garage door/gate opener functionality only. The '08 X has temperature and compass on the EVIC so no need to spend extra for that.
Fitting was easy - it is a direct replacement physically for the existing mirror and just needs a torx driver to remove and refit. Running the cable doesn't require too much effort as it just slips under the trim - I ran mine down the left had side (driver side) and dropped it out by the footwell.
With this mirror there are three connections, a constant live for the homelink, ignition live for the auto dimming, and a ground.
The constant live I piggy backed off the red wire on the diagnostic port connector which is +12v. No problem as the door opener takes very little current and is only operated when the button is pressed. The Ground connection is to the left of that and requires a 10mm socket to remove and replace the nut with the "install ready lug" slipped over the threads.
As for the ignition hot feed, that was more challenging since the actual ignition key doesn't appear to have anything over 5v (ie all signal wires = LEAVE ALONE) - best place I found was the violet/red on the back of the unmarked cigar lighter which is +12v when the ignition is on. Getting to it involves a bit of effort though.
You need to remove the center top dash panel (careful of the compass wires) then remove the two 7mm nuts underneath. Then remove the lower trim panel by the transfer lever (moving to 4L helps with access) and remove the other two 7mm nuts securing the bottom of the center panel. Gently pull it forward. Routing the cable is fairly straight forward along the inner lower edge of the dash below the steering wheel. The supplied cable was about 2 inches short so I made up a small extension cable with male/female spade connectors and a bit of spare wire.
Testing/setting up Homelink is as per the supplied instructions which are again very straight forward.
All in all it looks factory standard (I suppose it is on the Sahara/Rubicon!) Either way looks way better and adds useful functionality - so money well spent.