Advice for a undiagnosed rear leak on 99 GMC Sportsmobile

[99Fern]

New here and new to camper vans, hoping someone has seen this problem before and has pointers. Bought a beautiful 99 Savana 3500 Sportsmobile conversion. Exterior and interior is perfect, however I have a small leak near the rear cargo doors. Replaced the rubber gasket around the doors and the seal is tight. There is no visible rust or holes on the outside. Mechanic said its possible that it could e tailights or center breaklight. Before I go tearing up the custom upholstery and searching, just thought I would post here and see if anyone has encountered this and/or has any advice for a newbie!

[TomsBeast]

The center brake light is a common water entry point as the gasket shrinks with age, on the E series Fords. It's worth a shot to see if it fixes your leak