thanks for the replies
I've installed several of these in different cars, and one in my 00 saturn.
The main thing I was wondering was the stock engine shut-off system. My saturn had the GM Passlock system for which I neede to buy a module to override it, otherwise the car would kill the engine whenever you tried to remote start it without a key. I talked to my friend who works at cartoys and he said no such module is needed for the Celica. Good!
Yeah me and my sis are both aware of the dangers of leaving it in gear. She always leaves it in neutral with the ebrake on. And so do I.
I got an aftermarket system. Pyle-something. It's a two-way alarm with remote start

I'll let you know if I have any questions about the installation when I'm doing it

Never worked on a Celica before. I've always been modifying saturns for as long as I remember lol Looks like Celica should be fun. I plan on buying some more stuff for her to make it go a lil faster, so I'll be lurking around