If it were me, I would trace the wire harness in the trunk going to the lamp. A short is a wire touching bare metal, so look for a pinched harness as it comes up to the trunk from behind the rear seat backrest. Also look were the harness comes from the body inner fender to the right front lamp assembly.

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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.