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 ***** 1st time).
Fill a 5 pint/3l jug with hot water. Pour carefully into this area avoiding open A/C.
Leave to dissolve *****.
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)

 

This should be posted in the A8 forum. I should be a mod!

 

agreed, well about the move part.