Transmission harness?

Last year I bought a basket case of a XJ (a '99) for a song. I've managed to fix nearly every issue with it, however, I cannot get the speedo to work. It throws an P0500 error code, and as a result, I can't get the XJ to pass smog in Illinois.

I believe my issue to be an electrical issue, as I know the speedo gear makes contact with and spins with the shaft in the transfer case. I'm considering replacing the transmission harness on the vehicle, as the driveshaft ate the rear O2 sensor wiring once upon a time. Is this plausible? Can I replace just the transmission harness? Is it modular or would I have to replace the vehicles entire harness?

Troubleshooting steps done for the speedo:
Replaced speedo gear
Replaced speed sensor
Jacked up rear wheels and spun them, watching the speedo gear turn with them.

I don't have experience with this specific harness, but if you think it's damaged, (ie, an open circuit somewhere), could you jumper the signal wires from the sender to wherever they terminate? Ie, if the signal goes straight to the speedo, then run a set of test wires between the sender and the cluster.