The dash-buried evaporator core is a notorious W140 failure point, reportedly riveted to the firewall so that replacement requires pulling the entire dashboard. Quoted costs range from ~$2,800 at the low end to $4,000-$10,500 at dealers, with book time around 24-25 hours. …
AC evaporator core failure and labor-intensive replacement
Mercedes-Benz S-Class (W140) · mechanical