In the trunk, look for signs of water droplets landing on the wheel wells and running down. If you find these "tracks", for sure you have a bad rear window seal. Very common on the W123. The only water visible will be in the trunk, unless they get bad enough to show overflow marks at the bottom corners. You can pull the rear seat bottom cushion, too and check for water there. If you find any, rear window seal.

Other places to check are the sunroof drains (they go down the door posts, and if clogged, will run water around the base of the door onto the floor pan).

Bad door gaskets sometimes allow water underneath, too.

Front windshield will also leak invisibly down the firewall and under the carpet.

AT this age, I would suspect both front and rear window gaskets are bad and require replacement.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!