After finding this forum whilst on the hunt for interior lighting information, I found the socket pictured. It was located above the glovebox, wrapped in electrical tape. There are three wires leading to it from a loom behind the centre console.

 

[URL=http://thumbsnap.com/dmKrXVW4][/URL]

That is the plug for fitting a passenger airbag deactivation switch-

Ahh, thanks for that. Not something I need. Was hoping it might be something to do with the Aux input on the stereo.

The aux cable is usually only fitted if the socket is present.
It's very rare to have the cable but not have it connected.
You can buy the aux cable quite cheaply off eBay but tbh the sound and connection quality is not much cop.
I used one of these and ran it to come out under the demister switch panel originally but after fitting a dash cubbyhole it now comes out a slot where the lighter socket light would shine through the ashtray insert.-

http://pages.ebay.com/link/?nav=item.view&id=300471701053