You can check if you have this rattle by putting your left knee up against the lower left dash panel while going over a bump. If your rattle disappears, this is most likely your fix.
The only tools you'll need is a small metric allen wrench and a plastic pry tool (or screwdriver wrapped with electrical tape).
First, remove the lower trim piece. This is accomplished by pulling the small black trim from below the steering wheel (it's the lower 180 degrees of the steering wheel) by just prying it out. It has two clips holding it on. It should just pop off.
Remove the lower dash panel. Using a plastic pry tool, pop off the two covers on the panel, one to the left of the steering wheel, and the smaller one to the right of the steering wheel. The lower dash panel is attached by 5 allen head screws. Two to the left of the steering wheel, one to the right, and one at each lower corner of the panel which screw in vertically.
Once the five allen head screws are removed, the panel just drops out. The top right screw on my car was difficult to remove, so I just loosened it.
The following is exposed.
All that is required to stop the rattle is to tape the vertical portion of the bracket behind the dash panel. Add about 7 turns of tape to it as shown in the following picture.
Put the panel back in place and insert the screws. Remember, just get them all started before tightening any of them up.
Your dash rattle should be fixed! If you have H&R suspension installed on your A8, you'll be really happy because it rattled a lot before with that relatively rough ride.