Doing any mods to the sound system is opening a can of worms. You will have to install the head unit and a new amplifier at the same time. The stock amp will conflict with the aftermarket head unit. I have a aftermarket cd player installed, and only my 2 rear speakers and sub work. Luckly I got a 5ch 880W sony amp for $87 on ebay, and getting it installed today. ALSO! They don't make a wire harnes kit for the 1st gen gs300s. Metra makes dash install kit, which can be found on ebay. I'm not sure about finding it on a internet store. The stock sub is 10", I don't know the watts, the stock speakers are 4"(?watts) which can be swapped for 5.25", I've even heard of people fitting 6.5" speakers with some work and fabricating.