I don't know how common it is for other Explorer owners not having their outside air recirculation door vent, under the passenger side, windshield cowl punched out by the factory, but mine is not. In the link I provided, look at post
#15 and then the first two pics in that post. You'll see the under the cowl vent opening in those pics, not mine though. If I get around to it and feel like removing my cowl again this summer (just re-painted it 2 weeks ago), I'll take a picture of mine and post it in here. I would have done it before but I didn't think it was a problem for anyone else but me. It's really not a problem for me and I could always cut it out myself.
http://www.explorerforum.com/forums/showthread.php?t=377401
On the vacuum line diagram, see forum Moderator "Bigrondo", fairly certain he has the vacuum line schematic for our Explorers.
Edit: Here is the photo of my missing outer air recirc door intake that was never cut out on my Explorer. The pic view is from the under side of the passenger side windshield cowl cover. There is a riveted, rounded plastic cover over the fresh air intake opening and I have no idea who put that there. Possibly one of the two previous owners or a drunk Ford factory line worker?