Your '89 300SE should have a large encapsulation panel underneath. With it removed, you can access things like the alternator from underneath and the engine oil drain plug.
The panel is held in place by 4 sheet metal type screws that have an 8mm head.
I searched this sites parts dept. for one and didn't see it. This is one item I wouldn't have a problem obtaining from a junk yard. I suspect that if it's a dealer item, you'll pay dearly for it.
Here's a photo of the screws:
http://catalog.eautopartscatalog.com/smittys/sa/wizard.jsp?partner=smittys&clientid=sasponline.com&baseurl=http://www.sasponline.com/&cookieid=1LZ04S61F1LZ04SFTT&year=1989&make=MB&model=300-SE-001&category=Y&part=Screw