A method of forming a unitary perimeter seal for attachment at a fixed window panel of a slider window assembly of a vehicle includes forming upper, lower and side sealing portions each having elongated body portions and sealing lips extending substantially therealong. The upper and lower sealing portion are arranged with first ends adjacent a respective end of a first side sealing portion and second ends adjacent a respective end of a second side sealing portion. The sealing portions are arranged so as to result in a perimeter frame that circumscribes an opening for the slider window assembly. Corner regions are molded at and partially over adjacent ends of the elongated body portions of the sealing portions, such that the corner regions include a curved sealing lip that provides a transition between the sealing lips of the adjacent ends of the sealing portions.