Water from the sunroof
The sunroof is not intended to be watertight, the water that gets through is drained by four drain pipes. The front ones are behind the plastic trim on the A pillars, the rear ones behind the C pillar trim and they usually get blocked. Never try to clean them by rodding or blowing into them, they can puncture or separate at the joints. Suck the blockage out of the pipes using a vacuum cleaner instead.
To gain access you have to drop the roof liner: a solid panel to which the sunroof mechanism is attached. Remove all trim panels from the A and C pillars, remove the four grab handles, the sun visors (all these items are secured by screws). Gently lever out the roof panel from the metal trim at the back and let it drop.
Watch out for the plastic clips joining the trim to the body. They are slid into a holder on the back of the trim, then they are pushed into the bodywork. To release them, get a screwdriver behind them and gently prise them out of the body. It's easy to break the holder and cannot be easily repaired, however, if you only break one or two, the remaining clips should hold the trim panel in the proper position anyway.