I'm about to install 2 12" subs in the back of my car, but all i can worry is that there won't be enough sound from my other speakers. Right now i don't think my system is loud enough. And my question is would you guys recommend switching my stock speakers with new speakers or install brand new one's in the back. If i were to install new ones i would buy 2 speaker boxers and get 6" x 9" speakers and a new amp.
Put your focus on your front speakers. Some decent components if you can afford it would be nice, but you could always get coaxials. Either way I would upgrade your front speakers.