If you have a dampness in the passenger footwell, dap you fingers in it and taste it, if it tastes bitter(no not the alcoholic drink) could well be coolant from heater box/pipes going through bulkhead, if you get your head down in the footwell and look up you may see the pipes going through the bulkhead, if their is any green/white staining around them then they could be leaking.
Or pressure test the cooling system and have a look around under the dash before ripping anything out.
Also check around door seals and windscreen seals.