The screaming stage doesn't generally lead to failure, although I have had the imput shaft shear in the thinner worm section due to fatigue; I presume because of these type vibrations.
The speedo we were talking about I am personally familiar with. I used to work on them myself. My company now (for the last 15 years) has a separate radio and speedo devision and I don't see mcuh of the actual work on late model speedos. I can't give you disassembly instructions for them as I can from those old memories.
I can tell you that the speedo you have is as different from the old one as if one were a Chevy and the other a Ford. When we do speedo repairs now we disassemble most of the device, including removal of the needle. The old speedo didn't need this but many new ones can't be disassembled without removing the face. If you needed to do this you need to have a way of calibrating it when you reassemble.
You didn't say what your speedo was doing but if it is totally out you probably have a cable problem. This is very common in 190s as lazy techs pull the inst cluster out far enough to unscrew the speedo rather than unhooking it at the tranny first for play. The cluster can almost come out far enough but the cable housing (but not the cable) gets streched and now the cable is just not quite long enough.
Bosch Master, ASE Master, L1
33 years MB technician