I don't know if yours is in the same spot, but on my 88 300SE: Open the hood and stand alongside the driver's fender back toward the windshield. For lack of a better description, the engine area is separated from the windshield area by a verical metal "wall". In the area between the windshield and the "wall" you'll see all manner of relays. The OVP is the one with a clear plastic top that easily flips up. You'll see a single 10amp fuse. That's the OVP. On mine if you're on the driver's side it's the relay next closest to you from the infamous one stamped "KLIMA".

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Thanks,
Ron Brooks