The S model has the base radio installed, and while it has an AUX input function, an AUX input connection is not available.

Base Radio

It is however very simple to add this.

Installation AUX on Disco 3 S

1.      Remove the passenger side front kickpanel trim (unscrew the bonnet release handle first then pull firmly toward the centre console).

2.      Locate the grey multiplug which is the middle of the three.

Kick Panel Removed

3.      Unplug the 2 halves of the connector and carefully remove the blue, red and black wires from the block (they are held in with clips so a small flat blade screwdriver should do the trick). Note: These wires terminate here and are only used if the LR DVD system is fitted so you won't cause any problem by disconnecting them.

4.      Using a standard commercially available 3.5mm stereo extension lead (Maplins etc) simply solder the lead into the red blue and black wires. Red and blue are left and right channel and black is ground/screen.

3.5 mm input

5.      Route the cable to your chosen location (mine runs to the front of the centre console near the 12v lighter plug etc but you could run it to the centre console stowage box or anywhere conveniant)

6.      Re‐fit the trim plate, plug in the ipod or other aux audio source, enjoy

iPod connected

Source: http://www.disco3.co.uk/forum/topic2972.html?highlight=aux