Oh, the dreaded Evap leak on W140s...
What's the percentage rate of failure?
Believe me when I tell you is 100%!!!!
Yes, all evaporators fitted to W140s will fail sooner or later. There are a hundred tricks to slow down the process, but eventually, all have to be replaced.
Don't be so sure a NEW evaporator from M.B. will last you a lifetime. According to the trade here, the new evaporators still have the same design problem that made the original evaporators leak in the first place (a combination of copper and alluminum piping).
Only fitting a 100% copper pipping aftermarket evaporator could almost guarantee you that you will not have problems in the future. They are expensive, but when you take into account the money you save on labor by removing the dash everytime the evap fails, the price looks like a bargain.
CL 500, 1998
S 500 L, 1998
E 320 T, 1995