Question: "Why is water leaking under my dashboard?"
Answer: There are several reasons that we frequently run across. The first is that the drainage holes in the scuttle area in front of your windscreen are blocked. This area then fills with water until it can find a hole to leak through. Frequently this is a hole in the bulkhead where the pollen filter is located, but it could be a grommet, blanking plate or any other number of holes.
The second most common is from the sunroof drainage system. There are silicone pipes that run down the a-pillar to let water drain out behind the front wheel arch. If there is a blockage, if the pipes split, or if they pop off their connectors, you may well find water leaking out from behind the dashboard.
A third, less common problem is with the heater or AC unit. These don't get taxed quite as hard in the UK as they do in some hot countries where this is more of an issue, but they can spring leaks and spill fluid onto your carpets.