The floor mat and the carpet under the floor mat in the driver's footwell is quite wet and I'm looking for possible causes. The car has no sunroof, the windscreen has never been replaced and does not seem to be leaking, there is no loss of coolant that would suggest a leaking heater core and there is no evidence of water running down the plastic cover over the firewall.

Has anyone experienced this? Any ideas? Is there drain that is prone to blocking?