I suspect that your motor in the module is stuck. You can power it directly to see if it runs, I checked mine this way, push the rear brake and intermittently power the module, you should get an ABS like action. You can also check the sensor, but a scope works best for this. Check the short harness to the rear module, it is tucked into the space very tightly, there could be a break in the wiring that only opens up when the harness is twisted. Ohmmeter on each end and twist and bend might show this up. Other wise swap the modules and see if it codes on the front, not really that hard to do.
Brian
Canoe BC