Speaker Level Inputs

I'm trying to add an amp and subs to a 2004 Sonata but I don't want a new headunit. I've heard I can use the factory headunit with something called Speaker Level Inputs. What are they? and How do they work?

Any help would be appreciated.

You're trying to add a system to your car with an amp and subs to your stock factory headunit. I'm not to familiar with your 04 Sonata, but most stock factory decks do not provide any sort of rca out put.  You will need something like a converter. I would suggest a sound gate converter.   With this you will need to tap into speaker such as the rear, and connect this converter. Which then will give you an output option, and that would be were you will run your RCA's to the amp.  As far as ground and power i would assume you already know how to run those.   For your Rem you must  tap into your igntion turn on key.

For sound quality reasons...I recomend not to use the speaker level inputs.

A decent LOC with an impeadence matching coil, not just the cheap ones with only two resistors inside, is what you need.

At the shop I worked at, these sold for about $25, you can probably get it cheaper.