Hello, hope everyone is doing well. I’ve had a problem with my 2013 RX350 where any time I use the AC or the heat, water pools up in the carpet of the entire back row. Regardless of what vents I close or mode I select, there’s always a collection of water found spread out under the entirety of my backseat carpet. I’ve been to several mechanics that have no clue what the cause of the problem could be as most have heard of water pooling up on the front passenger side due to a clogged drain but never in the back row. There is absolutely no water in any other section of my car except for the back row. This only happens when I use the AC or the heat. All drains under the car and on the roof are clear. Anyone have any idea on what’s going on? Thank you in advance!

 

Welcome to CL and you certainly have provided a puzzler of a diagnostic issue.
It appears you are stating that the rear sunroof drains are clear. That they also made sure the A/C condensation drain line was clear and that you confirmed it by watching for water under the front middle of your vehicle.
That you have no idea where the water (or is it coolant?) in the rear comes from when you run the A/C or heat, which would be 98% of the time in my vehicle.
How long have you had it and do you know the accident history?

How much water are you talking about, sloshing around levels or damp stinky carpet? I recall my father's 1977 Audi 5000 and when you made turns so much water spilled out of the vents your pants got wet.
Do you see water coming out of the rear center air vent? Is it wet around it or under it?
Has anyone pulled up the rear carpet to look around?
Do you know if a heater line carrying coolant runs into the rear of the vehicle?

 

Thank you for your reply. Everything you have stated is correct. Upon visual inspection everything seems to be in order in terms of drains and connections. I’ve had the car for a little over a year now. The previous owner sold it to me after a tree fell and dented the rear section of the roof, which I got fixed by a private body shop. No prior accidents. This dent actually led to a whole other issue with water entering the back row due to a leak in one of the rear windows. After getting that fixed as well, everything was fine until I used the AC in the summer. And now I’m especially confused after seeing the problem persist when using the heat.

It is “damp and stinky carpet” in terms of water levels. It’s really only noticeable once the back starts to smell and thankfully it usually dries up rather quickly. The rear center air vent is dry and nothing but air comes out of it. The water is usually present in a way that appears as if it started from the center and pooled outwards. Also, the water is always cool to the touch, even with the heat on so maybe it’s coolant? Honestly I’m not sure if there is any kind of line carrying coolant that runs through the back. I lifted the carpet from the side just enough to reach around and dry out the water and couldn’t find any obvious source of the leak.

 

The coolant will be warm to hot to very hot after driving for more than 10 minutes. You can always check your coolant level in the engine compartment at the overflow bottle. If low or getting lower, then it may be coolant, if not then that source is eliminated. Also, the Toyota/Lexus coolant is Pink.

The rear sunroof drain tubes are very difficult to access from above and I am not sure where they drain, are you sure they were really cleaned out? Who did the work?

I suspect it may all be related to the tree falling on the roof. You may need a car leak specialist or body shop to check it out and check that window and door (and sunroof) for leaks. They\ leak specialists usually travel to car dealers to work on warranty leak issues. I have seen them place sprinklers to run over cars.
Or set up a sprinkler to run over the car and get in the back seat and wait, or have someone spray a hose over the vehicle where the tree fell while you are in the back.

Spray Lysol on your stinky carpet to help with the mold and smell. It really helps to lift it up and let air get under it to dry out and prevent mold which will make the entire car stink until the whole carpet is removed and replaced. Prop it up with something if possible.
If there is jute (fiber) carpet pad that stuff is like a sponge and it may be best to rip it out and buy some other carpet pad to replace it with once things are fixed. I have not lifted up the rear carpet so do not know what is under it. Being a "luxury" vehicle with more sound deadening I suspect there is padding under the carpet.