[XR5FoMoCo]

I have swapped out a front sensor to each of the rear positions, with the rears into front position - all 8 sensors work when in front bumper, but fault when in rear bumper.

I've now had car scanned at Supercheap Auto which returned a code B1B5201 pointing to a fault with rear Park assist speaker (as per Mondeo workshop manual description of code). I've always assumed sound came from car speakers...but rear one is located near drivers side passenger C pillar apparently (in addition to the front one in dash).
Will gain access in a day or 2 and see if any loose connections to begin with.