In addition to the above info, I'm thinking that the other mount is bad and is over-stressing the new one. I'd replace both at the same time.

My 1965 Mustang lost both mounts about 4 months apart.

Michael

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Usta haves '69 250/8, '76 280C, 1971 250C 114.023, 1976 450SEL 116.033
Current have, 1983 300SD 126.120