Some aftermarket amps have speaker level inputs which would work with your factory head unit. You can also get a converter which takes your speaker level outputs and converts them to line(rca). Not the best thing in the world to use, but if you aren't building a competition system right now they work fine.
My advice is to trash the stock head unit tho. You will notice a big improvement in quality with an aftermarket unit. my $.02
My advice is to trash the stock head unit tho. You will notice a big improvement in quality with an aftermarket unit. my $.02