96 Celica GT question

My Girlfriend has a 96 gt with a stock CD/Cassette deck in it. She wants the system that I had in my old car (Subs and Amp). First question is since I bought the line output converter cause Im sure the stock radio wont have pre-amp outputs, do I still need to remove the radio? Because I thought I will still need to connect the remote sensor to the radio. And if so how do I go about doing this because it says it has a security on it and it wont work if removed. Second question is where is the best place to punch through the firewall so I can run the main power cable to the battery? This will just make my life a little easier when Im trying to connect everything. Thanks to everyone who trys to help me out on this. I know some about installing systems but I don't know nearly enough. :headshake

"stock radio wont have pre-amp outputs"
Completely correct.
"do I still need to remove the radio?"
You don't have to if you don't want to, just wire it in in parallel with the new system you're putting in. But I would not advise having them both turned on at the same time as one may effect the other with very bad results. They shouldn't, but they might. I am not sure what you mean by "Remote sensor", do you mean the automatic ariel control from the radio?

You don't have to punch any wholes through the firewall. Stand in front of the car looking into the engine bay. On the right hand side at the back you will see a convoluted rubber boot right next to the windscreen wiper motor, and it will have a bunch of wires going into it. The boot is secured with 2 star studs so you will have to buy a star-stud driver to remove it (advisable as it can be a right pain trying to thread the wires through with it in place), they don't cost much so don't panic. You will find it nearly impossible to run the wires through the opening that the normal wires go in, due to it being just the right size for them and them only. Just cut a whole in the side of the boot and run yours through it. I have done this with my car for the amp power and a whole load of other things and it has been just fine for 2 years now.
I have attached a photo. The green circle shows you were I am talking about. The main purple line going in is normal, but you can just see the second smaller purple line going in on the right side.

:banghead: Oops, my bad. I have just realised that the photo is off a different engine to the one you'll have.
Just look around under the hood and you'll find a similar point where the loom runs through inside.