I've never had this issue, but my wife says that I have many "issues".

My thoughts would be since the turn signal control also controls the "flash to pass", it could be that...they have been known to fail...dealer or boneyard...

I could find no listing for a headlight relay.

The headlight switch could also be suspect...maybe a loose connection....

http://catalog.worldpac.com/ibusiness/JSP.v.1.01/wizard.jsp?header=http://egermanparts.com/header.txt&footer=http://egermanparts.com/footer.txt&partner=ipb&clientid=egermanparts.com&baseurl=http://egermanparts.com/&cookieid=7155611592PL0SPZF0&year=1991&make=MB&model=190-E-005&category=All&part=Headlight+Switch

Just a thought....

The Tenor Man

__________________
Currently Driving.....
1987 Mercedes Benz 300D Turbo - Daily Driver.
2004 Buick Rendezvous...Smaller than it looks!
2005 Mitsubishi Galant - Not bad for a tin box...Lousey Seats
Past Vehicles.....
1994 Dodge B-250 Van- GAS HOG..Succumbed to rust!
1989 Mercedes Benz 260E - Totaled but no one injured.
1977 Mercedes Benz 240D - Loved it, but SLOWWW! - Succumbed to RUST!
1976 Mercedes Benz 240D - Same as above! -
1975 Mercedes Benz 240D - Same as above!