Reversing warning tone

2004 X-Type 2 Liter, manual, Diesel.
Revering warning stopped working.
Does anyone have an idea what I can check please.

I think I found the problem. The reversing lights did not work either so I found the Reversing light switch and disconnected it and bridged the plug. The reversing light came on and the reversing sensors peeped. New switch is on order.