rails 16 18 the end stop 34 is configured to contact the movable window 14 and define an end point of the travel of the movable window 14 preferably the end stop 34 is perpendicular to the travel path of the movable window 14 as moisture accumulates in the guide rails 16 18 and the end cap 28 30 the drain 38 directs moisture away from the sliding window assembly 10 in order to promote drainage the drain 38 is preferably formed as an opening in the bottom wall 45 of the end cap body 37 as the movable window 14 slides down the guide rails 16 18 toward the end cap 30 moisture will accumulate at the leading edge of the movable window 14 and be directed towards an end cap 30 and the drain 38 upon reaching the drain 38 the moisture will flow down into the opening of the drain 38 being directed away from the end cap 30 by a spout 39 the sliding window assembly 10 is preferably mounted to a motor vehicle 40 and more preferably a pick up truck to readily and properly position the sliding window assembly 10 with respect to the motor vehicle 40 during installation a locator 36 and a flange 42 are provided as shown in fig3 and 4 the flange 42 is preferably formed by the motor vehicle exterior wall 41 and interior wall 43 the locator 36 preferably extends substantially perpendicularly from the outer side wall 52 of the end cap body 37 during installation of the sliding window assembly 10 into the motor vehicle 40 the locator 36 is.