Help requied in diagnosing PDC fault code

[Ronin]

I've had numerous issues with PDC in my A8.
I've replaced all front and all rear sensors, front ones still don't seem to work but they stopped throwing codes, i think maybe the speaker for the front ones may have gone?

With the rear ones I'm getting one code:

01549 Supply Voltage for Parking-Aid Sensor 29-00 Short To ground.

Could this be a wiring fault or maybe the PDC module in boot can be faulty?