the number of revolutions completed to determine the relative position of the associated latch truck 67 the rotary sensor can for example produce counted pulse signals each representing a relative position of the latch truck 67 alternatively or in addition a sensor can be provided on each latch truck 67 to monitor its movement along the guide rail 65 a rotary sensor such as a hall effect sensor or an optical sensor could monitor rotation of one of the wheels 77 79 of the latch truck 67 in the present embodiment the cover 41 has an auto detect strip 81 provided at the leading edge 55 of the cover 41 the auto detect strip 81 extends along the leading edge 55 of the cover 41 the auto detect strip 81 comprises one or more sensors not shown for sensing any obstacle inside the vehicle 1 disposed on the path of the cover 41 such as the back of the rear seats 17 or an item disposed in the load space the auto detect strip 81 is provided for determining when the leading edge 55 of the cover 41 encounters an obstacle or obstruction such as the back of the rear seats 17 the auto detect strip 81 can comprise one or more of the following sensors a mechanical switch an electrical contact or a capacitive sensor in the present embodiment a capacitive strip is provided on the leading edge 55 of the cover 41 the capacitive strip can be used in conjunction with the position monitoring unit described herein alternatively or in addition the current voltage drawn by the motor 69 could.