You aren't going to like what I'm going to say.
On the W140 everything from the firewall to the gearshift lever has to come out to replace the evaporator.
It is NOT a job for a do it yourselfer unless you've worked for Mercedes-Benz in an earlier life. The part itself is only about $350-400 but the labour runs around $3000. When you get it done make sure that they use the updated part.
Sorry