i have a au2 fairmont ghia and the speedo needle has stopped working?? but everything else is working okay all other gauges and lights are fine!!!

What could be causing the problem

It could be a number of things, the speedometer stepper motor could have failed, a component in the circuitry or it could be as simple as a sticky needle. Maybe the speed sensor on the gearbox, but that would affect other things on the car like transmission shift points etc.

I'd say it would be instrument cluster related. Maybe take it to an electronics technician to have tested.

I'm assuming it's an auto - Does it go up and down the gears okay ??

In the EF/EL's the speedo and tacho movements are interchangeable - AU2's might be the same - check the VDO part no on the indicator motor - if you regain speed after the change over, then the indicator motor is suspect.