Qs: Citroen AX 1.5D speedo/odo

The tachometer / mileometer / odometer / whatever you want to call it on my Citroen AX (1.5D) has not worked since I bought it. (However, the
speedometer works just fine.) I've been attempting to get it working again. I got the instrument panel apart and the instrument cluster out, and found to my surprise that there was no mechanical speedo drive cable in there - it's an all-electronic affair. I say surprise because my Haynes manual made no mention of this possibility (in fairness it does say not all features of the 1.5D variant are covered).
I can't see anything immediately wrong in there, however it's difficult to test anything when I don't have a wiring diagram to refer to. So I put it all back together, the speedo still works, and now the trip meter is working (which I could have sworn it wasn't doing before).
I noticed that the only connection to the speedometer / odometer cluster is a three-wire plug - one of the three is obviously ground, I'm guessing that the other two are maybe a speed signal and a distance- travelled signal? Anyone know? (I have a cunning/stupid plan to rig up a means of switching these lines in/out while on the move to see what happens.)
Also if the trip counter is turning but not the tachometer / mileometer / odometer / whatever, does that indicate that the meter itself is bust and the incoming signal is fine?
Lastly, I notice my friendly local scrappy has an earlyish model Citroen Saxo available, which looks to have a very similar instrument set, it's tempting to get down there and whip the instrument panel apart on the offchance that the speedometer / odometer cluster will be the same as for my late-model AX. Anyone confirm/deny.
PS Anyone who has ever managed to disconnect the final stage of the gear linkage from the gearbox on one of these deserves a medal. :-(
Cheers, Nick ( snipped-for-privacy@DELETETHISANDDONTSENDMEANYDAMNVIRUSESiee.org)
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