The rear window in the wagon uses the same thick rubber gasket that the rear sedan window uses -- these get old and hard and shrink and allow water around the glass and around the gasket to body seal. They are not expensive.
Ditto for the W124 wagon rear door glass -- mine is leaking already ( 1988 model) and will be replaced this year sometime. I get water pooling in the latch, no other signs of leakage.
When the windshields leak, they will drip water off the bottom edge somewhere -- for the rear, look for water tracks on the wheelwells. They can also dribble water down the rear seat back and onto the floor from under the rear seat cushion. Only occasionally can you see water tracks on the actual gasket -- it usually goes underneath.
The easy way to check is to see how pliable the gasket is -- if it is hard to the touch or you can move the windshield (or it creaks on bumps), the gasket is shot.
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!