The sliders just pull a cable that routes around a number of cogs just behind the centre console into the dash. Either the cable is seized, or a cog has come off/become miss-aligned (more likely). Something could also be blocking the air direction flaps, preventing them from fully opening.
If you take out the passenger footwell shelf and the centre console you'll see the cogs. To get to the flaps involves removing the air box thingy (bit of a pain).
EDIT: Haynes has got a sh#tload on this, so I would invest in a copy if I were you!