If it's a Gen 1, unplug the fan motor, remove three screws, fan falls out on floor. Mop up inside with something that kills ALGAE, then use flashlight and mirror to see what's blocking the drain. You could back-probe it from underneath, but it would still plug right away.
Change the cabin filter to improve airflow too.
There's a place in your signature where you can put your vehicles details, to save speculation.