it's not difficult. there is a writeup on the 123 D's, but not on the SD's. it's just so simple. pull the cluster, (if you have problems with your turn signals, now's the time to change the flasher...) pull the cover off, and pull out the speedo, then change out the gears that are garbage.
some advise using coat hangers pushed in around the cluster, but you can just grip it with your fingers and pull it out.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!