Nissan Primastar - renault trafic - Reversing sensors - Hugo {P}

Reversing sensors have stopped working altogether, yet the reversing light is working fully - so the relevent switches etc seem to be working.

Can anyone advise me where to look first for the problem please? They became faulty once before and during the last genuine Nissan service I had the mecanic found a loose connection and fixed it, at no charge to me.

The reversing sensors are quite handy on this thing, LWB van with limited visibility.