I just completed this fix on my son's 98 Ranger this afternoon. The original problem on his truck was that it was blowing cold air continually, rather than hot air. He had a faulty blend door actuator that needed to be replaced. This was tested by checking to see if the actuator arm moves when the temperature **** is adjusted. The replacement part is YL5Z-19E616-AA and costs about $68. Once this was replaced, the actuator arm operated, but we still had the issue with the broken piece in the plenum. We drilled 3 holes into the arm to find the best location. Originally, we used a nail with about 1/8" on each end as suggested by some of the other posts, but we were never able to get it just right. Finally, I ended up using a sheet metal screw and left about 1/4" exposed to the area that opens and closes the door. The other thing that seemed to be important, at least in our case, was to drill the hole near the end of the arm (away from the casing) so it reaches down far enough into the plenum to open and close the door. The information I got from here really did help alot with the fix and hope this can also help others.
With a little patience, you can fix this problem in a couple of hours and save yourself big $$$.