It's not the miles, it's the number of times that it has been turned off and on. Frankly, I would have changed mine when I had the dash out last summer if I had known that the random heat issue was caused by the switch. I am a believer in preventative maintenance, but I also hate replacing good parts. If it feels sloppy and there are not clear detents for each position, change it and avoid having the heat on when it's 106 in the shade. Been there...
The right one has a wire pigtail that matches the cable connector on the car. If you buy the wrong (964) one it will have places for you to solder the wires yourself, and it will cost more too. Sunset has the right one, don't let the dealer sell you the one that is the replacement number.