Most cars are wired up the same way as far as stereo stuff goes. Some are more sensitive to electical 'noise' than others. For most things, I've found that if I don't know how to do it, I try to learn as much as possible about a specific thing as possible. Currently I'm blasting away at grounding kits and grounding points on our cars.
For me, I usually just use google to find specific information and follow the 'web' of information. I'm not trying to shut you down, but your question was a bit vague for me to give a specific answer.
If you're looking to replace the stereo, it might be helpful to go look for installation guides for other cars that have more documentation, then apply what you've learned to the tC.
All modern car stereos are going to have at least the following:
positive lead for power
negative lead for ground
positive and negative leads for each speaker connection (2 front, 2 rear)
an accessory cable that remotely powers a sub or power antenna (this is a +12v connection).
Then you start getting into amplifier preouts, etc.
In most cases, you can find a wire adapter that will come with connections to a new stereo to the existing factory speakers. Basically, its a connector that has two different ends. One that matches the plugs on the new stereo, then it routes the wires into another connector that matches the factory speaker outputs. Keep in mind that this is for the most basic stereo connections. This is a basic install and requires you to remove just a couple screws for mounting. For multiple amplifier and speaker set ups, its best to route your own cables using a bunch of crimp connections and possibly solder. I haven't installed a stereo or alarm in over 10 years so things might have changed. But the one thing that I know has remained is the power, ground, and two connections to each speaker. Outside of that, its all about how creative you want to be.
As for the OEM sub, PM me your email address and I can send you a PDF of the install instructions. It's a 416kb PDF.
See if you can come up with more specific questions and I'll be happy to fire off better info.
