Had this problem with my W123 too.

I finally traced it down to the valance panel where the wiper vents were sitting on. What I did was to remove the wipers, its retaining screw rings, the two wiper vents (they are secured by 4 plastic pins each). The valance panel is taken out by removing 4 rivet-like plastic pins. Once the valance panel is taken out can you then see 15 to 20years worth of grime, leaves and Rust! Rust starts around the corners right below the windscreen and below the wiper motor mounts.

I derusted mine, sealed rust holes with polyurethane sealant (The type for windscreen seals), and greased entire area thoroughly. Refitting was easy.

Never had the leaking problem again!