The OVP relay provides power to the lambda control unit and the idle speed control unit. The input to the OVP is alway hot and not fused, so I think the fuse on the relay protects the relay itself.
If it is blowing immediately, I would suspect that the relay has an internal short to its ground. Other possibilities are that either the lambda control or the idle control have either an internal short or one of the feed wires is bad.
On the OVP, the feed wires come off pin 2 and are red/blue, The ground comes off pin 31.
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe