You need to get a small flat screwdriver and carefully prise off the lower panel that surrounds the clock and fog light controls. There's then (IIRC) two screws that hold the bottom part of the upper panel in place.
Next, remove the stereo (make sure you've got the security code etc.) and remove the two nuts either side of the opening in the dash. I think they're 6mm and are VERY easy to drop into the black hole of the dash.
I think that's it; you should then have clear access to the screws that hold the heater controls in place 8)