The odometer, fuel gauge and rev counter have ceased to work, although the temp. gauge is still ok. Any ideas?

check the fuses .. there is a power supply that works the instruments.. possibly a blown fuse.. with a mirror look under the dash for a wire that has disconnected.. this sounds like a ground has came off someplace. ok gilermo.

There r NO ground wires that r attached under the dashboard. Just hanging there waiting for u to find.

l have the 306 electrical wiring schematic in front of me now.
The fuel gauge & rev counter (tachometer) r on the same 5 amp fuse..#2 in the fusebox. Your car is a Phase Two 306, so it will have 30 different fuses in the fusebox.

Other things on this same fuse: brake fluid level, brake pad wear indicator, hand brake ON light, oil level, oil pressure, STOP! light. ARE THESE WORKING??

l cannot find a fuse for the odometer. However, the temperature gauge is part of a complicated Bosch or Magneti Marelli motor management system involving the water temp, air temp, gas valve, intake sensors, speed sensor, Lamba-sonde, fuel pump, etc.

There is a recognised fault with these dash boards its a connection fault remove the dash and carefully clean all the connections plugs and connectors on the dash