When I had this on my 1991 300CE, this was the problem:
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=24616EQV12470PRI69&year=1991&make=MB&model=300-CE-002&category=M&part=Horn+Contact+Ring
I first changed my voltage regulator but that didn't do it. I finally noticed that sometimes the flickering coincided with moving the steering wheel. There are also horn contacts that are very inexpensive that may be worn down. I changed these too but changing the ring did the trick. There was no obvious failure of this part. It fits on the back of the steering wheel.
glenmore
1991 300CE
2000 C280
1990 LS400