How to fix my leaking back window in my chevy silverado?

i recently bought a 2004 chevy silverado ext cab and found out the it leaks in water on the floorboard. i've pinpointed it to the rear window on the drivers side. it comes in there and runs down the side of the floorboard up to the driverside and soaks the carpet.. i think maybe its the seal.? think i should just try to put silicone on it or get it professionally fixed.. ? do you think the silicone will work without making a total mess.?