you have that head unit installed in your celica? but your not the one who installed it?
FIRST! Unplug the battery and wait 30 seconds before going to work!!!
Ok then you must pop out the consoleHow to remove Console
Once you remove that you should have clear view of your head unit, from there, just unbolt or un screw the bolts or screws around the head unit, that look like they are structural. pull on your head unit untill it comes out the harness will follow.
When best buy installed your head unit,they either striped and spliced the toyota wires (the hard way) or got the toyota wireing harness adapter(the easy way), which was stripped and ready to be spliced with your aftermarket head unit.
If your trying to remove your head unit compleatly. You can just unplug the adapter from the toyota wireing harness (it should be BLUE) and you will pull out your aftermarket headunit and wireing harness adapter. That should be on the left side of the head unit looking straight in. on the right side will be a black wire, thats the antenne you can just pull that out, any other wires.
Now to run your subs and crossover either there should be sub preouts, or if you want them running only in the rear so you can fade it back, you can run the subs into the cross over and amp then into the headunit through the "rear audio out" it should be next to your audio in for a changer or other device.
I hope i helped, reply or pm if you need anything else