Just realised yours is a '99 from your intro post - that won't have the mounting points for Isofix as we were still drafting the standards for Isofix at that point
There's evidence that post 2002 build cars should have the necessary threaded holes (2 on each side) to allow a bracket with Isofix anchorages to be bolted in place, at least on A4/6, even if they weren't fitted as standard. Didn't investigate A8 at the time. Given that my S8 is a 2002 and does have them, it seems Audi did start fitting Isofix as standard on the A/S8 earlier than some of their other models, as you'd probably expect. A lot of A/S4, A/S6 don't have them from the factory and for months Audi swore blind to me you couldn't retrofit - then I came across a post online with part numbers from an Audi parts guy and sure enough Audi changed their tune.....
What seat have you got? We're using a Maxicosi Cabriofix which allows the base to stay in the car with adult seat belt holding it in. I do find the Isofix anchors are in a strange position in the 8 - they're a bit low compared to the seat squab so you end up with car seat leaning back a lot. Not a problem for forward facing, but for rear when my son falls asleep his head tends to fall forwards onto his chest, which doesn't happen in the A6.