Usually from the rear window area, especially if rust below the rear window, or the seal around the tail light.

Secondarily the trunk seal.

Unfortunately it is a problem endemic to these W126 cars, most of which are now 20 years old or older.

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1991 560 SEC AMG, 199k <---- 300 hp 10:1 ECE euro HV ...

1995 E 420, 170k "The Red Plum" (sold)

2015 BMW 535i xdrive awd Stage 1 DINAN, 6k, <----364 hp

1967 Mercury Cougar, 49k

2013 Jaguar XF, 20k <----340 hp Supercharged, All Wheel Drive (sold)