i didn't know napa sold the plastic part. i ordered mine from an audi parts supplier, so i guess it's possible they have the right one.
it's not too hard. just a pain. lay on your back, upside down in the car - feet on the headrest and head on the floorboard. reach up, remove the electrical connectors, then with a teeny-tiny screwdriver blindly remove a couple of screws. wiggle the switch out, replace it and try to tighten the screws without being able to see anything...
pedro has a pretty good write-up. took me about 20 minutes to do mine. best tip - get yourself a really small screwdriver with hardly any handle to it. something that could remove the screws from a pair of eyeglasses but with a handle of no more than an inch, maybe a half-inch.
Replace Ignition Switch