For 900's (seemingly all), the speedometer gauge ground (ground reference) is the brown/grey wire off pin 4 of the 14-pin connector behind the tach area of the cluster. This ground circuit goes through one of the large 53-pin connector blocks (not usually a problem area) at the driver side A-pillar, tucked up in the rats nest rearward of the hood release. This in turn goes to the ground rail on the underside of the electrical distribution unit (relay tray) in the center console which in turn goes as a black wire to the chassis ground rail at the right A-pillar behind the ECU (again, not usually a problem area). If the relay tray has been recently disturbed then it may well be worth pulling it to check the connection under the tray (pull off, clean up if dirty, crimp a bit if loose, reseat -beware of live wires under the tray). The FAQ suggests the terminal contact in the connector on the back of the cluster is a possible problem area (again, pull, clean, crimp as needed). Additionally, you may wish to check the sending unit terminal connections at the cluster. They're at the aforementioned 14-pin connector: pin 3 (brown/white wire, sensor +) and pin 5 (green/white wire, sensor -). Otherwise, suspect the speedometer gauge, sender, cluster circuit board or other areas as suggested in the speedometer articles in the Electrical: Instruments section of the 700/900 FAQ.

While I've got the manuals open I'll clarify the FAQ for 700 owners (at least those with later VDO or Yasaki clusters). The speedo gauge ground reference is the green/black wire off pin 11 of the 12-pin connector behind the clock area of the cluster. It goes to the left A-pillar ground chassis rail behind the kick panel (right A-pillar for RHD cars).
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Dave -940's, prev 740/240/140/120 You'd think I'd have learned by now