If you have a water leak into the interior of your A8, this may help.
Lift the bonnet and check with a torch if there is standing water at the rear of the engine bay in the area betwen the E.C.U. and the A/C unit.
Lean over, point the torch down and have a good look.
This is the water that runs of the windscreen. It has to go somewhere and it takes tree muck with it.
Take an aquavac with a fllexible hose and gaffer tape a 4ft length of hose pipe to the end of it.
Suck out any water/leafs/tree muck by putting the hose into all the areas where the water from the windscreen could get to (I got 1 gallon of ****e 1st time).
Fill a 5 pint/3l jug with hot water. Pour carefully into this area avoiding open A/C.
Leave to dissolve ****e.
Push hose as far as you can into unseen areas to vac out.
Repeat process several times and finally you should see water pouring from the bottom of the engine bay.
This worked for me and was the last resort before paying audi to remove the entire heating system to access the drain holes (how much! ;oh my God).
The downside is that you cannot see that you have actually unblocked the drains.
The upside for me was that I sorted out my car with no help from anybody.(Including this site)

 

You can accesss the bottom of the drain from under the car. If u look straight up on the left & right side of the trans you will see the drains. Fortunatley I had a lift. Used compressed air to unblock the drains. Also dont forget the condensor drains on the left & right of the console inside the car. You have to remove the lower trim panels on the front of the console. More than likely they are clogged also.

 

I am having the water leak problem now for the third time. I had water streaming in the passenger area of the car (speciffically over the brake and accelerator pedals, passenger side next to the center console, and the rear console where water ran out in both passenger foot rest areas.) The first time I had 3 inches of standing water in my car after a serious rainstorm. I vacumed the carpet for 4-5 hours then went to a mechanic who cleaned out my drains with CO2. The second time I had water collecting behind the center console which eventually found its way out. The third and finally time was closer to my first experience than my second one. I took the car to Audi (who has my car now) to fix the problem. Does anyone have a way of preventing these drainage backups? I can't believe that I have to cut every green thing down to save my Audi...

 

Is the water leak problem a common one for Audi or just the A8?

 

Good question. I just found my 2006 A6 sedan with water in the driver's footwell last night. We had a torrential rainstorm. Now that I've read these threads, I'll have to look about.

I am a bit astonished and disappointed that water can get inside the cabin at all, with all the doors and windows closed. This is my first Audi, 3 months new to me; Not a good start...

 

All audi models suffer from water leak problems, on the A4, A6,and A8 the most common is the drain under the battery gets clogged up and water finds its way through the pollen filter box into the cabin.
Sunroof drains leak often as well.

 

I figured I would find something on water leaks here... I've had my 99 A4 for three years now and for the second time this morning, the floor behind the drivers steat had collected a good inch of water. We had some record rain today here in Long Island, I've never seen anything like it. But, the first time it happened there was an empty water bottle in my back seat so I chalked it up to it spilling out. WRONG!! I checked both driver side door seals and they were good and all around the sun roof felt dry. I'm going to look into the ventilation system now, I didn't even consider that... Crazyness!!

 

As I was making a turn in my 1997 Audi A8, my foot got a cup worth of water spilled on it. It was a shock for me, considering that this was once a $100,000 car. My guess is that water overfilled in the cabin filter compartment, then made it's way through the vents around the car. This is bizarre! My entire center console is soaked and my carpet is filled with water. It's too dark and too cold to do anything about it now, but I will clean the drains out first thing tomorrow morning. I wonder if the sunroof drains need to be cleaned too? I'm thinking about using my pressure washer to clean everything out... I wonder how this would turn out?

 

Good how-to here audipages Unclogging the A8 Evaporator Drains

May also want to check the sunroof drains.
In the Bentley they talk about using long flexible plastic rods to clean the sunroof drains.

 

Jake,
Thanks for the article - this has saved me a lot of time and headache! Cleaned all of the drains and am crossing my fingers that the problem won't return anytime soon. While I was at it, I replaced the rear sun shade (automatic) - feels good. I was surprised at just how much junk I found underneath the rear seat from the previous owner.......