if you are really on a tight budget is suggest that you get a nice 4 channel amp with front and rear crossovers that is bridgeable (pwede gawing 2 channel lang)

then what you do is use the stock front speakers of your car, driving it off the front channels of your amp, crossed over at around 80 hz. then you bridge the rear channels and drive a single 10 inch sub using the bridged rear channels, crossed at 80hz and below.

most of the newer cars have fairly okay speakers, its just that they can't handle the low bass at loud volumes. if you let them play 80 hz and above, they will be fine, kahit laksan mo pa.

andy