Blower motors on Audi's are very poor, I replaced mine last week and they
cost alot of money, they are made by veleo, the fuse on the A4 is a 30 amp
one, I don't know about the A6.

It's very easy to replace on the A4, so I imagine it is on the A6, the
glovebox comes out with 5 x 6mm hex screws, behind the glovebox is the
blower motor unit, its fixed to the housing by 3 small gold hex head screws,
there is also a relay that can go, but I doubt it has, take the blower motor
out and remove the wiring, use a mutlimeter on the 6 pin plug and check you
have voltage, there are 3 12v supplys when the fans should be on, and 1
variable supply that ranges from 1v -6v this is the fan speed feed.

The reason the blower motors go so often is the climate on Audi's is on all
the time, this usually burns out the blower motor after 4 years.

It takes 5 mins to remove the glovebox, and 10 mins to remove the blower
motor, so give it a try, you can't do no harm.

going back to the fuse, if it's like the A4 it is located in the fusebox
which is accessed via the drivers side panel between the door and console,
just pop the panel off and look at the diagram on the back of the panel you
removed, the fuse is a 30 amp green, but I doubt very much it has gone, a
little test is, remove the fuse then plug it back in and see if the blowers
work again for a short while, mine did.

Ronny