Does anyone know if there is a Cabin Air Filter in a 1991 C4, and a drain hose to drain any condensation inside the cabin evaporator case, and where they are. I have spent hours on line but have found nothing available, nor in my Haines repair manual. Any help would be appreciated.

#1 ---Order a factory service manual for your make/model car.

No cabin air filter. Your smell is most likely mold forming on the A/C evaporator. Definitely check A/C condensate drain hose. You can possibly see it from the top, but it's easiest to get to it from underneath, if you reach up next to the exhaust pipe on the passenger side of the car(make sure the vehicle is COLD...don't want any heat tattoos).
GM has a TSB on odors from the A/C system. It involves cleaning the evaporator, and installing an "after-run" relay that will turn the blower motor fan on after the car sits for a few minutes after parking. The system DOES pretty much eliminate the odors, BUT....it also has a tendency to drain the battery eventually. I've disconnected plenty of those dang things to "fix" a parasitic battery drain that couldn't be found by normal diagnostic methods.
The cleaner used on the evaporator core, however, is good stuff. It normally comes with a long, flexible applicator to get down into the core to clean it. Check with your local GM dealer to see if they have it in stock...they should. The TSB covers ALL GM vehicles(except some Cadillacs, since they are factory equipped with the after-run relay) from 2016 back.