A simalar thing occurred and is still current at least when it rains
Check my post on What MB Doc and other knowledgeable persons had to say

http://www.peachparts.com/shopforum/showthread.php?t=154730

But an additional factor comes to mind, (I do not possess a ML320 workshop manual neither have I actually seen the ABS switch) but I was once told that one of the common reasons for the ABS light to come on in some "domestic" cars is becuase the switch corrodes out and moisture gets in and it shorts out,