The buzzer is behind the dash. The instrument cluster has to come out. The part is not too expensive but, it takes some effort to replace it. If you can live without it, you will save some money. As far as the OVP, it could be a number of things wrong and if the problem is intermittent it may be difficult to test it. However, based on the symptoms alone, everything points towards the OVP (like everybody else is recommending). The only way you can be 100% sure is if you can take it to your mechanic for a definitive answer but, even then, if it is not a good mechanic, he may miss it. Whatever you do, DO NOT TAKE IT TO A MERCEDES DEALERSHIP, if you want to save money.
Good luck!

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J.H.
'86 300E