I noticed another clue:

For about the last 6 months, whenever I engaged the cruise control, the 'halo' around the speedometer would be intermittent. I chalked this up to the LCD going bad in the cluster.

Well ever since I cleaned the contacts in the CGW, the halo has been steady whenever the cruise is engaged.

Not 100% sure if this is a definite correlation but if you are getting this symptom on your W211 I would check your CGW to make sure it's not getting funk between the pins. This needs a T25 to take the lower cover under the drivers side dash down (3 screws).

I would recommend disconnecting the battery first before unplugging the CGW as it sets a ton of events in the system.

Still haven't gotten a chance to look for the water leak but it's been parked under cover since I discovered the problem.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech