Get some non-chlorinated brake cleaner. Spray the channel and the glass
liberally to get all the crud and grit off/out of it. Let it dry.

Now get some silicone sealant (caulking) in a small tube (not the big kind
you'd put into a caulking gun). Permatex makes various colors of such
sealant. Lay a bead inside the channel, or inside the molding itself if
it's constructed that way, then press the strip back in place. Stick it
down with duct tape for a few hours. Make sure it's all stuck down smoothly
and evenly. Remove duct tape when the sealant has cured. Pull the tape off
carefully from the middle to the outside, and angled down towards the
glass.