Remove the cluster and Look
Remove the cluster to find the real problem with the non moving odometer. I found mine in one of my Benz's with a broken end piece where the steel pin slides through the assembly numbers. A dab of JB weld on the end and it operates like new now. Generally the trip odometer can cause you havic if that's the pin piece that is stuck and then has broken. Take a look at the back of the speedo with it removed and you'll soon see the problem. If you aren't comfortable with removing this component than I suggest you send it to VDO to do the repair. Personally I like to keep as much of my hard earned green in my own pocket.
"Tell me and I will listen, Teach me and I will learn, Show me and I will accomplish, Involve me and I will succeed."
'84 300SD 256,000 Gold on Brown (Mileage Award)
'86 300E 246,000 Blue on Tan