Well,
I did some serious disassembly tonight, and I think I have the trip odometer working.
I submit this to the forum, and expect corrections and gaps in my knowledge...
Tools required - a variety of small flat-bladed screwdrivers. twezers, light oil/grease for relubing as you see fit. Clean work area where you can not lose stuff on the floor

Symptoms - Trip odometer would not reset, odometer otherwise counts just fine.
Culprit - small spring-loaded clips inide of each of the trip odo wheels - the spring that catches these loses its spring and fails to catch them onto the "trip odometer reset shaft" (my term) when it is turned - therefore, it can't be reset.
Take a look at my lousy-quality pictures

Overall view of the speedo -

FIRST - Pull the speedo from the bracket and put it on your clean

workbench
NEXT - Take a picture of the front of the speedo - so you know what the odometer reads and can make sure that it is correct when re-assembled. Yes, you can roll these odometers back if you take them apart, but there is some strategic inspector's ink so it'll be obvious if someone has ever messed with a speedo once you have it open.
NEXT - You'll need to pry the black metal ring away from the white plastc speedo housing - if you use something hard and plastic, you'll probably do less damage than I did, but since this was my first time, I just hacked it up. its also on the back. The goal is to 'loosen' the black metal ring that captures the glass to the metal speedo housing.
Pry the black metal open (from the back) until you can get the glass off.
good time to clean the glass right now.
NEXT - Loosen 3 screws from the back - they hold the plastic inner-housing to the metal outer-housing. - pull the insides out and set the metal housing aside
NEXT - Remove the circlip that holds the reset knob onto the reset shaft - its the only circlip visible from the back of the inside of the speedometer.
NEXT - Pull the face off od the speedo innards - it is held on by 2 small black screws and the needle. The screws are straightforward and the needle is tricky. I put a few pieces of electrical tape on the gauge face (to protect it) and pried up from 2 sides with 2 small screwdrivers - the needle popped off nicely. Might be easier if you have a soft-jaw vise, or good thigh muscles...
NEXT - seperate the mechanical innards from the plastic housing - I think that its just 2 screws that do this - see this pic (still has metal installed):

Loosen ONLY the long screw and the fillet-head screw (in this pic) - leave the other (small ones) alone, I dont know what they do

NEXT - loosen the screw and rotate away the worm drive that connects the speed input shafts to the main odometer shaft - spin this back and forth and admire the quality german engineering

NEXT - remove the 3 (4?) screws that hold the plastic trip odometer gears to the rest of the innards. One screw holds on a sheet metal plate, the other 3 go through the plastic into threads in the metal. Free the trip odometer from everything else. its ok becuase the trip odo works off an idler gear between the ones digit of the main odometer, and that turns the tenths digit of the trip odometer (which turns the rest of the digits via the small cogs(?) on the small side shaft)
You should get this:

NEXT - You need to remove the reset shaft - as you can see at this point, the reset shaft turns a brass bevel(?) gear, that turns the "trip odometer reset shaft" - ie: the shaft that goes through the middle of all of the trip odo gears.
This is tough, and I broke a tooth doing it. My method was something like this - hold the brass gear with one set of pliers, hold the trip reset shaft with another pair of pliers, twist them apart until the shaft is free of the gear - be careful not to lose small parts (now he tells us


NEXT - Once the reset shaft is free, the remaining shaft (the one with the gears and numbers on it) will simply pull out.
Suffice to say, if you do this, you'll end up with pieces all over the place, but just put some more electrical tape across the numbers and slowly pull the shaft out - be careful and work in a place where you can accidently lose stuff on the floor and still find it (I'm in a hotel room....)

NEXT - Seperate each digit, flex the spring and re-assemble.
You'll see that the trip odo reset shaft is not a cylinder - it has an axial slice taken out of it -
(more next post - JcG)