Have you checked the voltage along the VNet cable. This sounds like it is an electrical troubleshooting situation and you will need to trace all the wires.

If you have 12/5v at the USM then you should be able to check that the sensor and its return signal are working. If the signal is coming through to the USM from the sensor then it sounds like the cable from the USM to the dash.

Have you installed a terminator cap in the correct place?