do the harnesses use the same clips on both the DSM and the toyota? most factory audio components use harness clips on the speakers instead of having normal contacts, so you gotta make sure of two things:
1- they are the same size.
2- they use the same type of speaker harness clip.
Actually, the stereo that came in my GTS is pretty good, It's got nice bass for a stock stereo, minus a sub. If I put a single 10" sub enclosure in the cargo area and a small amp, well, I think it'd sound pretty damn good. Besides, I have a factory CD changer in the back, so it'd be a pain in the ass to swap in a new head unit (you need an adapter to use the factory changer)
the stock speakers in the celica are pretty good to begin with, and if you're one of those lucky kanucks that has the high-trim models, you got a wicked stereo from the beginning.
Hell, my car came with tweeters stock! not many cars have so much thought put into the audio system as the celica, it's well designed to begin with.
If you're gonna upgrade, get all-new aftermarket speakers, and a decent sub/amp combo. don't put factory speakers from another car into yours, that's just lame. The only way to truly improve the sound quality is to build your system from the ground-up, not hack off of other factory setups.
Just my two cents...