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The old blend-air actuator with the stripped gear:
The Dorman 604-206
On this column-shift, no-console 1998 Sable, access to the blend-air actuator is fairly good. The actuator is located here, but all work is done from the left & right sides.
Here's line drawings of what you're facing up under there:
Working from the driver's side floor, three of the five 8mm-head screws can be removed. Two retain the steel shield and are easy, the other is up fairly high. I used a 1/4" drive ratchet, and a std. & deep socket.
Now work from the passenger side. Remove the sound-deadening panel below the glovebox. Although the diagram below indicates three retainers, mine had only two. They pry out. They're designed to be one-time-use-only, so plan ahead and pick up a couple of new replacements. 1/4" dia. seem to work OK.
With the sound panel out, access to the fourth 8mm-head screw is quite good:
To access the last/fifth screw, you'll need to empty the glovebox and swing it out of its cavity. Pull inward on both sides, to get the two stops inboard, then swing it out.
With the glovebox out of the way, get your flashlight up there, move the wire bundle aside, and the last screw becomes visible.
With all five screws removed, the actuator pulls out easily.
The actuator's shaft is asymmetric. I found that when installing, I had to move the blend door lever by hand against the spring a little, in order to align the shaft & socket.