My 2000 Avalon did not have a filter installed when I purchased it but don't worry, it is configured for a cabin filter to be installed. The procedure to install a cabin filter is no different than for the newer models. The instructions come with the cabin filter itself (Toyota part # 88880-41010).
1) Remove undercover. 2) Pop off the airbag harness cover with a screwdriver. 3) Pass the airbag harness cover through the opening. 4) Remove 3 screws and 2 bolts holding the glove box. 5) Remove trunk opener switch connector (if applicable) and remove the glove box. 6) Remove the filter tray and replace the filter (hold the left and right knobs at both ends of the filter case at the same time and pull out the knob to the front). 7) The filter must be assembled with the "UP" mark on the filter facing the same direction as the same mark on the filter case.
Right answer: do not mess with the airbag(dangerous!!) or yellow /white cables. open the glove box and let it fully open by squeezing the tabs that hold it up and any cables. you will see a slot that the filter goes in, usually has tabs that keep it shut. open the cover and remove the filter. this particular model may have to remove the sorrounding cover, just remove 4 or 5 screws that hold the sorrounding glove box assembly and reassemble.