Based on this drawing, it looks like the two little plastic lenses that cover the bulbs in the overhead map light housing are held in place by the friction of the two little tabs on each lens against the light housing -- very much like how the single lens on my 00 LS400 is held is place:
http://www.toyodiy.com/parts/p_U_2002_LEXU...KAQKA_8121.html Carefully pry the lens out by inserting a flat blade screw driver near a lens tabs. Or use a small plastic trim removal tool if you have one.