How can I fix my Seat Alhambra's parking sensors?

I own a Seat Alhambra TDi Tiptronic SE. The parking sensors have stopped working and only emit a short continuous humming sound when the ignition is switched on; I looked in the fusebox for the symbol for the parking sensors' fuse (letter 'P' with a graphic symbol of soundwaves) but it does not appear on the fusecard. The fusecard has these two sets of digits printed in its upper right-hand corner: 7M3 010 258 C and: YM21 42A089 AA

I recently had to change the fuse that goverrns the headlamp washers; we had been in Germany and Austria one month ago, when the parking sensor problem started, so I wonder if maybe the extreme cold has caused a problem (-20 degrees). The water in the pipes feeding the headlamp washers froze and thereby expanded, causing the covers that normally lie flush with the bumper surface to rise up. I guess that their fuse may have blown because the headlamp jets' mechanisms wouldn't move because the water had frozen. Maybe the parking sensors' fuse has blown too, but I can't find it; incidentally, I have not struck anything and damaged the sensors, and they look fine. Is a fuse or relay hidden away somewhere? The Owner's Manual doesn't tell you anything useful, and the symbol for the sensors listed on page 3.67 of the Manual simply isn't shown anywhere on the fusecard 'map' of fuse locations and functions. Help!