1997 Cadillac STS evaporator replacement

Per vehicle diagnostic at pep boys, there is a leak on the evaporator, and needs to be replaced. They wanted o charge me in the $hundreds to replace.
where is the evaporator? How do I replace?

when I last charged the vehicle with freon, and after a few minutes of running ac wih freon in it, there was a hissing sound and a white cloud of freon around beween radiaor and firewall of driver's side.

does this sound like the right diagnostic. Thanks