Engage the emergency(four way flashers) and walk around the car. You will probably find one or more bulbs either illuminating dimly or not at all. The ones that are dim or inop will cause rapid or erratic flashing because of an open circuit or poor ground in the bulb socket. I advise to use silver base bulbs, not brass. Different bases will cause different resistances which can affect operation.

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81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.