My 89 leaks around the rear headliner and drivers side headliner area. Pulled the rear seat and water under the drivers side rear seat.

Are there common area's that water would leak in on a 87-91 Delta 88/Bonneville/Lesabre?
It has no sunroof and a vinyl roof. I've tried clear silicone around the front windshield, sides of roof and vinyl seam in the middle but still leaks.

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1989 Oldsmobile 88 Royale Brougham Regency - Grey on Grey - FE3 - X06 Package - Limo Tint - Clarion HU - 2K Pontiac Grand Am 15" Alloy Rims

Not familiar with leaks on these models. I would check the usual suspected areas like the door seals, windows. You might have to pull some window trim off to get a good look. There could even be a problem with the windows sealing to the body.

Living here in the NW with a lot of rain and I have yet to have a leak in my 89.