A slider window assembly for a vehicle includes at least one fixed window panel and a movable window panel that is movable along an upper rail and a lower rail at the at least one fixed window panel between a closed position and an opened position. The upper rail includes a channel, and a filler is disposed in the channel of the upper rail. The filler includes a filler body having opposite side walls, and the filler includes lips established along respective ones of the side walls. The lips are extruded onto and along the filler body. The lips include a compressible and flexible material and provide a dampening and cushioning function at the movable window panel when the movable window panel is disposed in the channel of the upper rail.