I've noticed quite a few posts for this problem with no resolution.

** Major job.... don't attempt unless you have a heated shop and about 5 hours.**

1. Drain cooling system.
2. Unhook heater core lines in engine compartment.
3. Take out top screws of dash (under vents)
4. Unbolt dash braces under glove box
5. Unbolt dash braces under LH dash
6. Remove light switch
7. Unhook ingition connector plug (behind light switch)
8. Remove Glove box door
9. Unhook vacuum line bulkhead connector below glove box.
10. Unhook large electrical bulkhead below glove box.
11. Lift dash out of place... should use an assistant. (Be careful not to break the rear view mirror.
12. Unhook vacuum line from firewall to heater case and bottom case bolts
13. Steps 14-20 can be skipped if you want to cheat...
14. discharge A/C system.
15. remove vehicle computer
16. remove coolant tank under hood.
17. remove bolts to connect a/c case to firewall
18. remove a/c case
19. remove bolts & nut from firewall side that hold heater case in place
20. remove nut from above distributor (holds heater case in)
14-20 cheat: chizel out 2 bottom bolt flanges from the heater case from inside cab. Firmly jerk the case away from the firewall. Note the case will crack and break slightly on the upper right side. Have JB weld ready to repair the cracked case.
21. remove heater case.
22. remove 5.5MM screws and split the case.
23. replace defrost door GM# 52470781
24. Epoxy broken pieces & put back together.

Job is quite the B-tch!!! Good Luck