i guess wiring.
years ago i changed the instrument cluster on a escort mk4 diesel to one with a rev counter. this was done by rewiring the connectors at the back of the instrument cluster.
i dont know what a fiesta azura is but i can say if it really old (pre 1990ish) and its a diesel then sender wire for the rev counter wires to the alternator if petrol then it goes to the coil. if its slightly new (1990ish to 2007 any thing with a ECU) i would guess the sender wire for the rev counter goes to the ECU (this is only a guess as ive never wired one in to a car with a ECU)
last thing i can say, im quite sure that on the old cars (pre 1990) a diesel rev counter is different to a petrol rev counter because when i was trying to find the sender connection on the alternator (diesel) i was testing it with a old petrol rev counter and could not get the it to work but when i got a rev counter from a diesel car it worked fine.