Heater boxes are located on the right side of the firewall as your looking at it from the front. there are bolts/nuts on the firewall, remove them. under dash should be the heater box, carefully remove it after all bolts/nuts are remove. make sure you remove all the hoses.

The best advice I can give you is to buy a factory G.M . shop manual available from a Co. called Faxon. You could also go to your local library and have them print it off of a Master copy. Trust me,it will be well worth having this information in hand.