From this drawing, it looks like the rear seats remove similar to my 2000 LS400:
http://www.toyodiy.com/parts/p_U_2000_LEXU...EAQFA_7102.htmlPry up the rear seat cushion by forcing your fingertips under the leading edge down by the carpeting. It can take some force to lift the seat cushion up and out.
Once you get the seat cushion out, you will probably see most of the bolts holding the seat back in. If it's like my LS, some bolts could be hidden behind leather flaps held in place by velcro in the area of the headrests or behind the armrest. After you have removed the bolts, lift pull/push the entire seat back upward without rotating it -- it may hang from hardware along the front of the package shelf.
I suspect you will find some existing holes -- at least my LS had a few and I pushed a wire to the trunk through one. If your GS doen't have an existing hole, maybe you will have to drill one. A diagram on the above website makes it look like the gas tank is below the rear seat instead of behind the rear seat back like in my LS. If you have to drill be sure you verify where the gas tank and other components are located so you don't have a disaster on your hands. Good luck.