This is a sedan.

I've had a water leak in this area for a while; finally dealing with it. The water is visibly dripping from the area of the inside trim at the upper rear corner of the left rear door. The door is well aligned to the body, fits firmly. I thought it was the inside weatherstripping (fastens to the body around the door opening, so I replaced that. I noticed that I definitely had to close the door firmly after installing it, implying that it's sealing against the door.

Alas, I'm still getting plenty of water in: a section of newspaper tucked into that corner of the seat (where the water had been hitting after dripping off the trim) was soaked after a recent rainstorm.

There's no visible body rust.

I've had factory cross-bars more or less permanently mounted, and the base of the cross-bars pushes out the weatherstripping mounted to the door. I thought I might try removing the cross-bars, and maybe even taping that area shut temporarily, to see if the problem is the outer weatherstripping. But, before I take that step (removing duct tape is always such a hassle!), any other possible entry points in that area for water? For instance, there's a trim strip running along the roof; do the fasteners for that trim strip penetrate the roof, and do I need to learn how to remove that thing to re-seal any such penetrations, or is there a way to determine if that's where the problem lies?

Those cross bars are meant for temporary mounting, not permanent. Yes, they will deform the rubber seal. Spraying the seal with conditioner and using painter's tape rather than real sticky duct tape might also work.

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Keeping it running is better than buying new

Well, yep, removing the cross bars and lubing up the weatherstripping so it fit tight against the body solved the problem. Thanks for your advice.

Instead of using duct tape, try a piece of tin foil - wet the area and smooth out the foil.

I had a left rear door leak after using load bars and used foil for a bad series of rain storms. ('96 855R)

I had a slight drip down the center of the window.

I cleaned and lubed the rubber parts with silicone spray safe for rubber.

After a week, I removed the foil and found that the problem resolved itself.