Can i add to my sound system in my car without changing my stereo interface?

I want to put some sub woofers in my car and get some new speakers and an amp. I have an 03 Honda Accord Coupe. I%26#039;m trying to find out if i can do this without having to change the interface on my dashboard.|||The CHEAPEST way is to splice RCA%26#039;s into your rear speaker wires and then run it to your amp. my brother has always run his like that and there are no problems. IF your amp has the high input thats the way to go but if not the splice will work fine. the inside wire is the + and the outside wire is the -.|||no you gotta get a head unit. you need the RCA jacks in the back to hook up an amp which will then be hooked to the subs. speakers u can get away with still having the stock head unit but if u want them louder, get a head unit that pushes out more watts (x4) and if u want even more get an amp for them|||No you dont have to buy an amp. most audio stores as well as walmart sell a device that will give your factory head unit rca preouts. check out this link http://www.walmart.com/catalog/product.d鈥?/a> it works on al cars and will give any stereo rca preouts. it a cheap way to give your factory some preouts to add an amp or two and a couple subs. i have one in my jeep and it works pretty well. i would recommend it lol . . . as a matter of a fact i just did|||Yes there are a few ways to add amps to a factory head unit. The least expensive ways are to use a line output converter (they are available at any local car audio shop or Best Buy), it basically turns your full range speakers into RCA outputs. Or if your amp is equipped with it you can use high level in (which basically means that you have a line output converter inside your amp). The other way is definitely more expensive, but is going to give you a much better sound quality is to use a factory correction piece such as a JL Audio Cleansweep or the Alpine Imprint. I know there are others out there but those are the only two I have ever used. I personally like the JL piece a little better, but I%26#039;m sure people would disagree with me on that one.