A lot of guys ask me what is best for their cars, where should they put their speakers, how much should they spend on their audio for a decent sound, bla bla.. I would try to explain this for everyone who is/has planning/planned to upgrade from stock, or those who are upgrading from their previous setups..
First thing, The Stage.. I'm sure no one likes to listen to a concert facing opposite to the speakers (with your back facing the stage), this explains why every audiophile puts speakers in the front.. That's why, you need front door speaker first, upgrade them. A component speaker would always sound good as compared to a full range speaker. Google Component/Full Range Speakers for cars. Do your study.
Second, an amplifier.. This is the heart of your system, that's why integral part of any audio setup. You'll find a lot of Chinese brands in the market with tons of nobs on them. I would suggest, first buy components, then save for an amplifier. Always buy branded. Cause Chinese would never satisfy your SQ urge.. Google an 'Amplifier for car', study LPF, HPF, Bridging Mode, Gain control, RMS, Power Input.
Third, a sub-woofer.. In order to enjoy the full spectrum of sound (High/Mid/Low range of frequencies) you need a sub-woofer. To workout a sub-woofer, you need a wooden box known as Enclosure (Usually sealed/ported), you put your sub-woofer in this custom made box which depends upon your sub-woofer and your listening taste. Google, sub-woofer, enclosure (sealed/ported), SVC, DVC.
Last but not the least, a 2.1 setup built using front stage, an amplifier and a sub-woofer, can pretty much satisfy anyone's requirement.
A stock setup can never be as good as an after market one, therefore, when you spend millions of rupees on your new car you should spare some healthy budget for your audio setups.
That's it..
Cheers!