From memory:
Turn the head unit on
Press 'PROG'
Press the round button below the '>' arrow on the screen
Press the round button below the 'auxin' (I think) option
Press the '<' or '>' (next/prev track buttons above the volume control) to turn it on or off.
If you don't get an 'auxin' (or similar) option, then check if you have the 'CD changer' icon on the screen. It'll be next to the single circle icon (which indicates a CD is inserted), and will look like two overlapping circles. If this is present, then the unit thinks a CD changer/ipod/USB is connected, and won't enable the aux in socket.
From what I know there 2 versions of the SE3xx head unit that were used in pre-facelift Mk2 Leons:
- SE359: Non-MP3 unit, with 'CD' button, used in MY05 & 06 cars (
Pic)
- SE360: MP3 unit, with 'CD/AUX' button used in MY07 & 08 cars (
Pic)
Looks like your car got the aux-in centre console jack, but got an MP3 head unit without the 'AUX' label on the 'CD' button, so maybe there is a third version of the unit with MP3 capability but no 'AUX' input selection. Would be interesting to know if these unit have CD changer control capability or not....