Wiring a second battery is easy.
You can just wire them in parralell this will allow your charging system to charge both batteries (keep in mind that will be another load on your alternator so you'd have to make sure it's a powerful one. An advantage to this scenario is that there is very little rewiring to be done and you will extend the listening time of your stereo before you have to start the engine.
Another option is to keep the second battery separate and rewire you're stereo to it. This has it's advantages (you can run your stereo till the battery is dead and you didn't kill your starting battery.) the main disadvantage is you would have to charge the battery with a battery charger every 4-5 hours.
In the middle is getting a battery isolator - this is also the best "easy choice". You would install the isolator and rewire your stereo to the second battery. your stereo will only kill the second battery just like keeping them separate, but your charging system will charge both batteries.
The absolute best option for installing a second battery is to give it it's own alternator and keeping the two electrical systems separate. This option allows you to keep your stereo separate from the starting battery, And charges both batteries without putting an extra load on your alternator ( I recommend a 60-70 amp alt for the main system and for the stereo get one that puts out atleast 20 amps more than your stereo uses). This way requires alot of work, you have to make a bracket to hold the second alternator, and do alot of wiring. There is a company that makes kits for installing a second alternator you can find it at
http://www.caraudiomag.com
If you need more power for your stereo the best option is just to get a more powerful alternator. 60-70amps + what your stereo needs.
I'm currently in the process of adding a second alternator in my car, if you like I can let you know how it works out, should be done in a few days.
