Adding an amplifier to the existing factory system
Okay, so maybe you LIKE the factory deck, but you want to
simply add on an amplifier and some subs. No problem. I did
this to my own car before replacing the factory deck.
Since the factory radio is turned on and off by Class 2
signals from the body control module, you need to hook up a
switch to turn the amp on and off manually. It needs to be
hooked up to the remote turn on input on your amp.
The factory deck doesn't have RCA jacks on it, so you need
to buy yourself an amplifier with high-level (or
speaker-level) inputs. Lots of amps have these.. they're
not hard to find. Alternatively, you could buy a line
converter, which converts the speaker the factory deck into
a line-level signal for RCA inputs. If you're going this
route - buy a good quality one. Cheap (~$20) ones will
sound like crap. (Don't say I didn't warn you!)
You can tap into the speakers on the rear deck (in the
trunk) and run your signal wires from there, if you want to
put in a subwoofer. It's not the best way of doing it, but
it'll work. You could tap the signal wires for the front
and back speakers, at the deck itself. Then run both sets
through separate amps (or channels, if you have a 4- or
5-channel amp) and enjoy your new amp'ed sound
system!