[Fantastic]

Common(ish) problem I believe. As you suspected, water runs off roof/tailgate onto the nice big flat hinges and into car roof lining. Don't know about whether the part you mention works, I kept the seals on mine coated with Autoglym Vinyl and Rubber Care and it seemed to repel/keep out the water... or I could just have been lucky.
Just out of interest, cars parked facing uphill seem to get more than facing downhill - more water running off roof on to hinges.