if we're talking about the fifth generation celicas, then yes, it's very easy for water to get on the speakers. There are two main ways it can get in:
1) On the inside face of the window. This assumes that there may be a small gap or gaps in the seal along the top where the window is supposed to be flush against the roof/front pillar rubber weatherstripping. If water is getting in there, it will most likely be miniscule, and you will never feel it in the car. The water will seep through the gap, and adhere to the inside face of the window, sliding down between the window and the window sill weather strip, and into the door itself. Once it gets to the bottom of the window glass, it will drip down, and if the placement is right, it'll drip right onto the speaker. Now unless there's an enormous gap there, the dripping won't be too much, and obviously only when it rains or you're at the carwash.
2) On the outside face of the window. This assumes that the outside face of the window may not be flush against the sill weather stripping, and water seeps in through the gaps there. Once there, it slides down the outside face of the glass (inside the door) and at the bottom of the glass, it drips down to wherever, possibly the speakers.
If you look carefully at the bottom of your doors, there are tiny holes at the front and back of the metal skin. Those are to allow water to drip out onto the ground if it accumulates there during a storm.
SO! In conclusion, check the seals along the top of the glass and along the top of the door where it hits the bottom of the glass. If water's getting into the doors, it will be getting in along one of those two spots.
D