If it's happening during a downpour, I'd suspect something leaking around the windscreen. Use a garden hose to spot it, you might need to replace the windscreen.
If it's while driving, then perhaps there's a passage through the firewall that's not completely sealed (like when extra wiring is run through and then not sealed up with silicone). It would be worth adding a little Sikaflex to the area around where those wires enter the cabin.
If it's just happening because the air conditioning is running (and doesn't happen if the aircon is off), then there might be two possibilities - smell the fluid to know which one it is. If it smells like coolant, you might have a leaking heater matrix which will need to be replaced. If it's just water, check the hose that is supposed to pass through the firewall to let the water out of the air path. Some Navaras went out from the factory with this hose a little short, with the result being the aircon would leak water (and more when it's humid = raining) into the front footwells.
Enter your email address to join: