Stay away from the silicone. !!! Sealing the gap between the cab and plastic trim piece will do nothing but make a mess when you go to fix it right. Water can go down in there all day long, that isn't what causes the leak. It's supposed to go in there, and drain out the sides.
Just did mine a couple of weeks ago using the write up that JWO stated. The hardest part was dealing with all of the @#%$ silicone that I used trying to seal it up the first time.
I followed the instructions to a T the first time. Didn't get a good seal.
Took it apart and cleaned everything up again. The second time instead of using the sealant in the tube, I got a roll of Butyl tape at the local NAPA.
Worked like a charm, with no mess.
The Butyl tape is definately the way to go.