Hi Bob (Gawd, I love writing that phrase!)
Been there, done that. I've heard rumors of mechanics that can repair this problem without removing the dash, but I don't see how. They must be a whole lot smaller than I.
I pulled the dash to replace the element when it gave up the ghost in my old '84. That said, I didn't completely remove the dash - you only have to free it up enough to pull it out of the way a bit - this saves some time & effort. I think it took me about four hours to do the entire job.
Until I got around to dealing with the issue I just wedged the center vent open. This isn't too hard to do - just remove the glovebox, pull the rubber boot out of the way, and prop the door open. Here in Dallas we can count on requiring a/c out of the center vent until, oh, about November.
As for why this one fails, that's easy. Of the five actuators in this car, four of them are on the underside of the dash, easily accessible through the footwells after removing only the undercover to the dashboard. This one is just under the (black...) surface of the dash, right in the center. The sun can do a much more effective job of cooking it there, as opposed to the others. That and Murphy's law...