Theres an amp for the subwoofer in the panel. Most people want to replace the speakers first, but the factory head unit is 20 years old, built for basic needs, and powered for really cheap factory speakers. A new (or even a 5 yr old used head unit) is going to have so many more features to get the most out of your system. You can spend thousands upgrading it to really nice, but the minimum IMO is about $200. A new head unit for about $100 will let you fine tune in your speakers and even the most basic will have a little more power than a factory unit. You can get a pair of coaxials for $40, or a little more for a set of components. Two pairs total for front and back doors, would be a big improvement. Its really not difficult, If you've done it once you can do it all in less than an hour, maybe two or three if you have to learn the simple tricks. There is a thread on upgrading the rear subwoofer or disconnect it if not your thing. Depends on what your budget is, and what you want out of it will determine where you go from there.