ok i just put a new strut and mounting plate in my celica. i still got the same bouncing noise comin from it. think i got a bad strut? anything else i need to check for?

check your bushings..

well i replaced the mounting plate with it i thought that was the only bushing....

If you follow your rear suspension down, you'll find there are two stabilizer arms per side, the strut side could have a worn or loose thru-bolt, if not, follow it up to underneath the middle of the car, where the arms almost converge, there are bushings in each side that like to eventually squeak a lot, and I guess if the bushings were gone, you'd have a metal-on-metal knock. Just push up and down on the trunk (or hatch) when its parked and listen closely. Go from there.

stem bushings can be gone, springs are hella worn, etc.