For all you pro's who hate the idea. It will not sound terrible unless you use the proper speakers. You guys are correct Subs 8'' and above will not work properly. Bass drivers and speakers will work. I don't think this is a stupid idea from experience since I've done it. I have done it and have had it done for 2 years no problems no rattles. Custom work was all done on my own. It is a good idea if you do it properly and know what you're doing. You have to enclose them properly to make them sound well. I am using free air bass drivers in mine they sound great. They are inclosed in a custom box to make them work
On another note some of you who are trashing the idea so fast are correct I will only agree with one part though. The Celica rear deck lid will rattle and vibrate if you're just going to cut a hole their and put speakers in it. You will need to use some other type of material or re-inforce that plastic piece so it doesn't not vibrate since the the plastic deck lid it will cause flex and a ton of rattle.
Follow the guidelines for the specifications of the speakers/subs to how they perform at what dimensions so you get the quality you need in the sound.