My odometer goes on and off?

For the 1999-2004 model year confirm fuses F2.21, F2.34, F2.5, and F2.37 are good and have power.

1999-2004 MY fuse panel schedule:
https://www.allfordmustangs.com/for...r-swap-wont-fire-please-help.html#post2669271

Check the ground behind the center console. It is shared by a number of devices including the radio, GEM, cluster, ODB2 port. It's frequently messed up during a radio install.

Follow the multiple black wires from the radio is one way to find the ground.

Another option is to test the ground wires at the cluster connector. Look for the black wires and the black wires with a white stripe. Again best to use a test light that will "load" the circuit.

Note, testing a ground with a VOM may work if the ground is open. However the VOM method want be able to tell a good ground from a "weak" ground. Again the best method is to use a test light that will apply a measured "load" to the circuit.

Or use the voltage drop method.

Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-fo...perform-charging-system-voltage-drop-test.56/

how to fix intermittent cluster foil trace repair 1999-2004

Also possible this is a bad cluster printed circuit foil trace or bad solder joint. Check out:

Intermittent Odometer - A Repair - F150online Forums

Courtesy of Suek here's how to calibrate for a needle swap.

How to recalibrate your gauges - New York Mustangs - Forums

There are also companies such as United Speedometer and Electric that will perform "bench repairs". I have used their service before and have had good results.