<>

That chassis does use an OVP relay and it is located behind the panel at the rear firewall.

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1W70PZW4N20K0APK8I&year=1991&make=MB&model=300-SL-001&category=P&part=Overload+Relay

If you use the same Parts Catalog [at the top of this page-BUY PARTS], you will notice there is no OVP relay listed for your chassis [ 129.076]..that is b/c there is none.
Plug in your chassis model/year and then hit 'Chassis/Body Electrical" ..