Seat Leon Owner's Manual page 183

182 Driver assistance systems
Towing bracket
If the trailer power socket is occupied, the rear parking aid sensors will not
activate when reverse gear is engaged, or when the button  is pressed.
This function may not be covered under warranty if the towing bracket is not
factory-fitted. This causes the following limitations:
Plus parking aid*
There will be no warning about the presence of obstacles in the rear area.
The monitoring in the front area remains active. The optical display changes
to towing mode.
