Broken Odometer

The 1990 models do not utilize a speedometer cable; they are fully electronic, driven by a signal from the speed-distance sensor. A faulty odometer is usually caused by the stripping of the small plastic gears inside the speedometer. Over time the plastic becomes brittle and then cracks, eventually stripping or breaking in half entirely. Replacing the entire cluster would rectify the problem, however the instrument cluster is unique to the 1990 models and aren't as common as the earlier ones so your friend might have issue finding a replacement. Replacing the odometer gear itself is an option; this website http://www.speedometergears.com/chrysler-odometer-gears has a variety of gears, although you'll have to open your cluster and see what kind of gear you require.

I recently purchased a 90 horizon with the same issue, I tried a different unit with the same result, I opened the unit to find there are 2 electric motors one for the speedometer and one for the odometer, Is it possible the motors are bad or the drive signal is missing, is there a way to test this.