This should be in the sound section.....
Visit
www.crutchfield.com and find their 800 number. Call it, hold for 1 min, tell the guy what car you have. ask the guy if there is a "kit" that'll allow you to add an amp without splicing the wires. If he says no, ask what a speaker level to RCA converter will cost. They should be about 10-20$. You'll also need to power the amp so you might want to look into a small amp power kit, 25$-40$ bucks. if you get the kit for the radio, aks them where in the car it hooks up. This will allow you to determin how long of a RCA cable to get. And then you can buy the cable while with crutchfield too.... damn they can be useful. I think you get like 10% off if another crutchfeild buyer refers you. PM me if you're going to do this and I can give you my info.
What kind of amp is your friend hooking up?