Coolant in rear floor vent w124 300td

[AdrianB]

Hello guys,
I have been investigating mysterious wet carpet in my 1988 300td. I noticed that it was wet around the heater vent under the front seat. Removed it and there was coolant inside the vent. Guessing that a leaky heater core is the only way coolant can end up here? Find it strange that the coolant doesent leak trough the vent pipes from the "heater box" and exit in the transmission tunnel. Can the leak come from somewhere else or is the only solution that the vent pipes are clogged and heater core is leaking?

Sorry for my bad english..
Greetings from Norway.