......I have a 6 CD auto-changer that sits in the bottom of a mounting box which has 3 slots.

Does anyone know what are the other 2 for?

I was wondering that too.....are there alternative units that could be fitted in there? Can these autochangers be daisy-chained together?

On some cars the navigation CD is stored in the boot, I wonder if the Octavia had its nav CD reader in the boot too before the arrival of the Columbus?

Its probably the same unit that is fitted to other VAG models where equipment is mounted in it.

I've wondered that before! I fit my mini tyre compressor in one of the other two

Would also go with the logic of it being a shared part, where possibly you might have a navigation disc unit situated there as well.