Water in driver's footwell
 

I recently bought a 2011 E70 Xdrive35d, got it cheap because it has a salvage title. One of the problems was that the carpet in the driver's foot well was wet. At first I assumed it was the sunroof drain but after pouring 2 gallons of died water down the sunroof drain I determined that that wasn't it. So I removed the carpet (what a PITA, seats and center console have to come out) and left it outside in the rain over night.
Turns out the water was leaking in between the firewall and the plate that holds the brake booster. There is some sort of gasket but I guess it must have failed.
I considered removing the plate and installing a new gasket but that looked like a huge job (no room to work in, bunch of wiring harnesses in the way, etc. so I applied rtv sealant on the outside (had to remove the brake booster for that) and painted it over with spray rubber.
It's been dry ever since.

Same leak but on my passenger side. There is a small rubber drain in the bottom of the windshield. One under the cabin filter, and one under the brake reservoir. 10 minutes of careful exploration and soft plastic brush, the drain holes appeared.
Water was filling the caverns and leaking through the wiring loom and plug. Water resistant, but not waterproof. I know this is a june tread. Thought I would respond to help if you have not found the problem. Or something for others to delve into.

both left and right compartments where clogged for me I hit the drain plugs with a coat hanger.until it came out under the car. Id say its a recommended maintenance item

Agree on the regular maintenance item. Be careful with a coat hanger, don't want to puncture that soft german rubber. weicher deutscher Gummi
I used a kitchen brush, and a plastic zip tie to dig and discover.
whipper sniper line works well for chasing plastic pipe.
Next experiment ??? melting the end of the plastic line, so there is a little plastic ball on the end, so it is not sharp and will go around a corrugated bend.

Good to know guy's. I will check mine even though they are not leaking... yet.

This SO needs to be a sticky.

Same issue years ago with an E39 I had. Plugged drains in this well area where the brake booster is. It would fill with rainwater and eventually reach the level of a wiring harness that passes through the fire wall, into the cabin. Took a lot of investigative work before I figured this out.

I agree that this should be a sticky too...

On the vw tdi door seals we had a thin rubber drain outlet that would clog up a lot. The solution was to widen the hole. I wonder if we could do the same with these drains that clog up easily.