I did mine about two weeks ago, water trails in the wheel well indicated it was the driver's side the water was getting in (UK car, right hand drive).
I took off both the roof rail seals, cleaned thoughly, They were full of algae,moss, slime and other crud. I didn't remove any old sealant as I didn't want to create new problems where there weren't any.
I could see a hairline crack in both rail channels starting at the rear hatch, going forward for about 18 inches.
I sealed both channels with Nemesis grabs & seals, this was £4.89 from screwfix, this can apparently be used on wet of dry surfaces, I've never used it before, just read the reviews and took my chances, I can always scrape it out in future if it doesn't work.
I put on a fairly thin bead on and smoothed it in with a finger dipped in soapy water, after cleaning the seals, I refitted and so far it's remained dry inside.
Unusually for Britain, we haven't had a big downpour since to give it a good test, need to wait till I wash it (could be sometime ha ha ha) I'll update if it fails.