If you remove/lift up some of the trim panels in your boot or around your spare tyre area, you will see a wiring loom that (from memory its on the passenger side but dont quote me on that) that feeds through a grommet under/near one of the tailights and this is the loom that runs to the sensors in the rear bumper.
Anyway, have a poke around in the boot compartment and you should find the loom.
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