It's at the firewall, the top is red and you can see the 2 replaceable fuses.

https://www.google.com/search?q=Mercedes+M103+OVP+relay&num=20&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiPsK3Kne_aAhXszFQKHdYkDrMQ_AUICygC&biw=1645&bih=1200#imgrc=FDtQZaKvZJGDYM:

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1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).