The OVP has a fuse or two in top - depending what revision you have.
They are not $ 200.00
You can get a second hand one cheap from that great auction site to test out. If this is the problem you can get one here - forum sponsor for $80.00

see this : http://catalog.peachparts.com/ShopByVehicle.epc?q=1991-Mercedes--Benz-300e--Base-Body--Electrical&yearid=1991%40%401991&makeid=63%40%40MERCEDES+BENZ%40%40X&modelid=6195%3AMBC|1507%3AED|10000013%3ASM|20%3AEB|613%40%40300E+Base&catid=241959%40%40Body+Electrical&subcatid=242003@@Overload+Relay&mode=PA