Hey Nashville! Me too!
Wow, you seem to have covered everything.
Normally I'd bet on a ground at the bulb holder (brake lamp). My W210 ('96) had a similar problem (ABS/ECS + DEFECTIVE LAMP, whatever) I sanded my taillight contacts, too, then I removed the whole assembly and could see that the metal "runner" carrying the ground had overheated and melted the pin holding it on so it barely touched the lampholder contact! I bent the "runner" out and fixed that one. Shortly after that the dashboard lightshow came back; turned out the 3rd brake light had a contact that had arc'd away - not anywhere easy to find mind you, but where the housing's contacts slide into the assembly mounting slots!
One other time (no ABS warning though), it turned out one front side marker's filament had broken then reattached shorter, making it brighter! In your spare time you may want to "ohm out" all the bubs (hillbilly for bulbs).
Good luck, if it doesn't work out we can run head-on into each other fer the 'shurance!! (KIDDING- Ilove my car - and my karma).
-Tom

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RIP "Betsy" (Oo=*=oO)
'96 E 3 2 0 (W 2 1 0) M 1 0 4 (Totalled)
Spaces in sig so as not to screw up the SEARCH; every time someone searches for that MB they don't want my sig!

2004 Audi A8L
'98 VW Passat 1.8T 5M
'87 Alfa Romeo Milano 2.5L 5M
'67 Impala convertible, 327cid