Water pan is not something that I am familiar with, but if you are getting water on the floor on the passenger side then your heater core may be leaking, the drain tube for the ac is blocked ( it sticks out through the firewall on the passenger side and can be cleaned out with a coat hanger or compressed air).
it may be caused by the drain hole from the window screen being blocked it is located under the battery also check that the rain bar on top of the pollen filter is still there and not been left off allowing rain to travel in and down the heater pipes.
Rain is part of the water cycle and occurs under many varied conditions, all of which may condense liquid water from gaseous water vapor.
under the bonnet passenger side bottom of windscreen under plastic rain cover deflector whilst your doing that clear debri from rain drain both sides good luck
Rain
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Water wells are come from under ground water and its contain dirt and rain water.
Does it leak on the passenger front flooor?
The rain shadow effect is when there is an area of land that is blocked from precipitation by mountains.
Get ahold of the service manager and tell him your problem. Ask him if you can take it to another garage and have him pay for it. They just aren't checking their work. If it leaks while or after raining, I would suspect the windshield is leaking around the edge causing the water to run down the windshield pillar behind the trip. If it's not caused by rain, the air conditioner drain pipe is blocked, bent or missing causing the runoff to come out the bottom of the module and onto the floor. I suspect the heater core.
water
I had the same problem in my echo and I am guessing that it actually isn't a rain leak. If it is what I think it is you probably had a passenger kick off a hose that is connected to your condenser and the water is coming from your air conditioning. The hose is supposed to lead the water through the floor pan and out the bottom of the car, hense why you get a puddle under the car when you use your A/C. So look as far forward as you can on the passenger side floor and see if that is your problem. If it is actually a rain leak sorry for the big waste of time.
The more the rain the more chance of flooding, drains get blocked due to intense water pressure and so on...