The problem is the resistor boot under the coils..very common on this last of the Waste Spark ignition DIS systems.
Change all 3 of them. [ one under each coil /DIS system]
Leave the plug wires alone until you change these 3 coil connectors.
Techs change these automatically when doing plugs change on 104's b/c of their failure rate.

http://catalog.worldpac.com/mercedesshop/sophio/quote.jsp?clientid=catalog.mercedesshop&cookieid=2AO123CS42BD0O5777&baseurl=http://catalog.peachparts.com/&partner=mercedesshop&year=1999&product=F1010-84561&application=000357360

http://catalog.worldpac.com/mercedesshop/sophio/image.jsp?title=Spark%20Plug%20Connector&url=http%3A//img.eautopartscatalog.com/live/F101084561BER.JPG

I would also change the plugs while I was there b/c these great systems still take the Bosch NON Resistor Super Core plugs and they are dirt cheap..use NO OTHER but F8DC4 in 104 DIS engines..they love this plug

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=2AO123CS42BD0OD395&year=1999&make=MB&model=S-320-001&category=F&part=Spark+Plug

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A Dalton