It's very inaccessible i.e. you can only get one hand to it, which is really handy when you're trying to plug the changer in.
Here's a pic with the trim panels removed, just so as you're sure you don't have it.
Fitting the bus cable will take the job from 5 minutes to an hour or so, and even pulling the HU can take a while unless you're very gentle and use the right keys. The keys are designed to both free the clips and pull the HU out. Whatever you do don't pull the HU out using the keys as this can scrunch the clips and then it's jammed in there.
Remove the glovebox (which will make life easier when running the bus cable anyway) and with the keys in place push the HU from behind.
You may also find that you have a solid 20 pin plug monopolising all 3 of the sections that feed the DIS, provide the pre-outs to the rear amp and also the CD changer. If this is the case you'll need to either cut the end off the plug so you can plug the blue changer lead in, or you'll need to extract the pins from the cable and insert into the existing black plug. Another brilliant Audi design I'm afraid. Hopefully you've got the multicoloured plug, so you can just slide the new blue section onto the end of it.
Here's the solid black plug:
Here's the multicoloured one:
