Well, the solution just occurred to me a few days ago.
Firstly, I still occasionally get the flashing rear defrost light, but the charging system was perfect otherwise.

Well my blinkers are irratic (hazards too), my intermittent wiper work 5% of the time, and my rear defrost gets low voltage....
The connection??

It's all in that one combo relay!

Part No. 000-821-08-63 is selling for only $41.77 in Fastlane! I'll be ordering that now.

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2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)