This is a task that can be done. First remove the back seats, both top and bottom. The bottom pulls straight up and then outward, then you will see four bolts to remove the back seat. Once bolts are removed, pull up on the top portion of the seat and remove. Then pull the top speaker cover straight outward toward the front seats. This is held by four or five metal clips.
Once everything is removed, remove the 2 rear speakers, not the sub, if equipped. Once removed, you need to place a piece of MDF (medium density fiberboard) approx. 1/4 inch thick, nothing THICKER. This is where you need to cut each piece, one for each side approx, 9 inches wide and 14 inches long. This is a guess. I cant remember exactly. Once cut, you need to place over the factory hole. The wood should lay flat back there on the factory metal before ANY cutting is made. Thats where these dimensions come in. It will lay flat, once cut right. This is just a close guess. Trim as neccassary. Once the wood is cut and will lay flat over the factory metal, place aside.
Grap the new 6x9 speaker, and draw on the wood, both pieces, the outline of where it need to be cut for it to fit snug. I used a jigsaw for this. Do both pieces.
Once done, place the new cut piece of wood, onto the factory metal once again, where the speaker lays flat over the factory whole. This is when you take a sharpie or something and mark on the metal where you need trim for the 6x9 to fit. You dont need to cut the entire area, just enough metal trimmed for the 6x9 to clear. Do this for both sides.
Trim away the metal now marked, be careful not to cut yourself or the tint on the back glass. Its kinda tight. Once done, place the wood and speaker in place to assue it clears with enough room. Once it fits without any problems, you can screw the wood down into place with metal screws, then use liquid nails for the edges. Thats what I did, or you can place the speaker in the wood and screwd own first, then place the speaker and wood on the factory metal then screw into place. I didnt want to mess up and punch a new hole or damage the new one. Before you screw anything down, tie in the wiring to the new speaker. If you need to know the + or - for the polarity, check out this link.
http://www.lextreme.com/misc.htm. This is a great website for anything.
Enjoy your new sound. This was done off of memeory, so be aware. I hope this helps. Lastly, check for rattles, and fix it before you place everything back together. The factory speaker cover should be close to centered for the sound to be good. Double check to assure the centerness before making cuts on the wood. Keep that in mind.
Mark
1992Lexus