What part of the AC system is under the dash & & How hard to replace? on 1997 GMC Yukon

I've had my AC service and a lot of parts replaced. Now I'm told that something under the dash needs replacing and it will cost $1200. What ist it and can I reach it?

Asked by for the 1997 GMC Yukon
You can reach the evaporator core if you have knowledge and tools and patience. It is a lot of labor to get there.