I rolled this (mirror + installation) into my purchase of my '06 AC. The list price from the parts dept, if memory serves, is somewhere in the $250+ range. If you want to install it yourself I'm sure you can find it for less from an online vendor.
The installation instructions are available in the knowledge database for subscribers.
One came in my 02 regular cab, so I pulled it out before I sold it and kept it for my new truck. It was real straight foreward to install. Basically you just use common sense and tap power from any accessory line. It doesnt draw that much power, so it doesnt really matter where it comes from. It might have taken me an hour max to install.
:ts: $99 to under $250 is reasonable and from your comments we have nothing to fear about installing it ourselves. Our '00 AC came with one already installed and everytime the family's in my truck someone looks up to see the temp and it just ain't there....
I guess I'll have to think up a new project after this one's done.