I can't tell you for sure how to get the cable out of the housing. There may be complications in the form of a speed pick off for the cruise control if your car is so equipped. I think I remember it being on the firewall under the floor mat but it's been 10 or more years since I last worked on the W116 450SEL.

You will need to pull the cluster to get at the end attached to the speedometer. The cable may pull out from the top or it may be attached with a clip or ? at the other end.

What chassis is your car? I have the W114/115 and W116 CDs and can look it up if it's either one of those.

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