[SolRaven]

I don't know the history of the vehicle, but if the glass was changed without changing the seal that might be the cause. if you don't want to gunk it up with sealer, and are up to it I would suggest removing the window and seal and putting a new one in. while you have it out you will be able to see whats under it, who knows what you will find. if you do decide to remove it be vary careful that is an expensive piece of glass.

I will also ask if you have had a substantial leak back there. i had one case where the vertical body lines below the rear window started separating. if this is the case you should remove the window use c-clamp vice grips to close the gap and have an experienced welder braze them together, and possibly the bed line while your at it. Brazing shouldn't leave nasty beads or holes like a mig welder.