How to clean heater evaporator on 1992 grand prix gt

I have a 1992 grand prix gt with a 3.4L twin dual cam engine and it has approx 90,000.00 Miles. The blower motor seems to be working just fine at all speeds but there isn't any air coming out of the air vents. I checked with my mechanic and he said that the evaporator is dirty and needs to be cleaned inorder for air to flow through.But he said that inorder to clean it you have to pull out the whole dash to get at it. I am looking for some more info on how to clean it with out removing my hole dash or if there is any other way to get at it.

Thanks