I can't remember off the top of my head if the '04's have the same seal. I know they have a similar cowl design. The directions here are for up to '03, so you might have to just use it as a guide as some things may not be 100% the same.

The OnStar antenna on the roof is also a very common problem. A friend of mine had the same problem and the dealer just replaced the antenna and it fixed the leak.