after market is the way to get and make sure you get some new door speakers too your going to need them if you get after market i highly recommend pioneer deh4900 or higher
You can if you buy a line level converter and simply run tag lines off of the rear speaker wires. But to achieve maximum performance and sound quality I would recommend an aftermarket stereo with RCA outputs and a professional install. Or at least someone very knowledgeable in car stereo installation. Good luck and happy booming!!