The problem is the Combination Relay, which is located under the black panel (6 screws) just to the rear of the main fuse panel in the engine bay.
This relay controls the intermittent wipers, turn signals, and rear defrost.


Prior to replacement of this relay on my 95 C280, I had the following symptoms:
1. Turn signal flash is irregular, or not operating at all.
2. Intermittent wipers worked totally randomly.
3. Flashing LED in rear defrost rocker switch when rear defrost was engaged.

The part number is: 201 821 02 47.

Good luck! The part is about $50, and is cheapest in FastLane.

__________________
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!)