all you need to add an amp to your stock radio is a line inverter. cost me 10-15 bucks at Ovation. but apparantly the installation of it involves taking apart the head unit, which ended up making the whole sub/amp install take a little over 3 hours. I would have just done it myself but i had to get it done by a JL certified dealer inorder to get the 2 year warranty.
Also, i think new speakers would be a good idea if your going to add an amp, cause it wouldnt take a whole lot of powerto blow them. get some quality speakers and amp and your sound will kick ass