On the W126 there should be two knobs on the cluster - one is for the clock and one is for the trip meter. I believe the one on the right is for the clock, but to be sure just twist it and see if the clock adjusts; the other one would then logically be the trip meter. You will have to push the knob for the trip meter; if yours won't push in then it may be jammed. Are the numbers advancing properly on both the odo and trip meter?

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280