Just went through this with the 108.
Unless you can reach behind and unscrew the cable from the speedo head, you'll have to pull the cluster. Next, you'll want to pull the speedo drive gear from the tranny before trying to remove the speedo cable from drive gear. There is a rubber crush ring on the end of the cable, and if it's been there 20+ years, chances are good it is fairly stuck to the drive gear. Mine was, I thought I could just wiggle the cable, break the seal and be done. WRONG! I succeeded in promptly breaking the split ring retainer.
Pull the drive gear, then try to separate the cable. A 10mm bolt hold the drive in, and a 10mm bolt tightens the retaining ring.
Once all is loose, from the engine bay, pull the cable from the firewall and lift out. I forgot to mention, the cable may be held to the sub-frame by means of a small clip, just pop the cable from that clip.
If you have tranny fluid or gear oil in the cable housing, you'll need to replace the drive gear shaft o-ring inside the drive gear housing. It's small, just a couple mm in diameter.
To install the new one, start from the engine bay. Slip the speedo head end through the firewall. The new cable will come with the firewall grommet already attached to the cable housing. Get it in place, then attach to the speedo head. Reinstall the cluster. Route the tranny end of the cable through the exact route the old one was (the replacement is the exact length from speedo to tranny. No excess). Install the drive gear on the end of the cable, the place the drive gear housing in the tranny. Tighten the bolts and your ready to roll.
Use caution when trying to route the new cable. Avoid hard tugs or tight bends so as not to damage the new cable. Getting the cable routed was the time consuming part. Took me three tries to get it just right. It is fairly tight quarters along the side of the brake booster, through the steering shaft and tranny linkage (if column shift) and kick down switch cables.
Hope this is clear enough.
BTW did you ever clear up the vibration on the old gal?
[Edited by MikeTangas on 03-28-2001 at 01:21 AM]
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.