Remove the squarish carpeted cover on the side of the tunnel hump in the passenger footwell. To remove it you depress the center pin of the 3 plastic rivets approximately 1/8-inch with the tip of a phillips screwdriver.
Remove the knee panel below the glove box. Look below the glove box hinge and you will see a row of 3 or 4 screws, remove these. The front edge of the knee panel is held to the firewall by a nylon grip. Pry the two fingers of the grip apart and slide the panel straight rearward towards the passenger seat to remove. Trickier to do than it sounds.
Remove the glove box. With the knee panel removed you will see a new row of screws beneath the glove box hinge, remove these. Open the glove box and remove the row of screws along the top edge of the glove box. Slide the glove box half-way out of the dash, remove the electrical wires powering the light, remove the hose which sends cold air to refrigerate the glove box, then slide the box out fully.
Remove the side cover of the HVAC box which sits atop the tunnel hump. There are approx 10 small (6mm?) hex screws which secure this cover. A coil-spring style of flexible socket adapter really helps here.
Slide the old filter out of the heater box and discard. Pull out the foam strip above the filter and discard too. Install new filter and new foam strip which came with it.
Replace all parts in reverse order.
I am assuming your car is not a 1999 or early 2000 which still has the factory original filter. If I am wrong, then step #5 has to be changed slightly. But with 2008 only a week or so away, I think my assumption is safe.
Have fun! Plan on allowing 45 minutes to replace the filter your first time.
Well, I checked your previous posts and see you do own a 1999 model. Chances are someone before you has replaced the original factory filter, but just in case, the revised instruction step is below.
5. If you have a single pleated filter, discard it with its foam sealing strip and install the new parts. If you have two filters (a thin pleated filter and a mesh bag filled with activated charcoal crystals) then you will need to remove both filters AND the plastic rack which held the mesh bag. The only way to remove the filter rack is to break it into pieces and yank it out! There are lots of sharp edges in there, so don't cut your hand and drip blood all over your interior. With the old rack removed there is now room to install the new thicker pleated filter which is impregnated with carbon particles.