Audio Systems: car amps, farad cap, inch woofers

Question
what does a capacitor do for amps in a car and do i need 1 i have 3 amps a capacitor a line driver what ever that is and 2 15 inch woofers in a car i just bought but its not hooked up and i would like make it work. thank you

Answer
Not hard to 'make it work'.
A cap will allow the amplifier to use more 'instant current' than if it were hooked to a battery/alternator alone.
You need to add up all of the current requirements for the amplifiers plus the rest of the car systems. If the alternator can provide this amperage, no problem. But you will need a larger alternator if the amplifiers and the car (added together) need more than the currently installed alternator can provide. You will kill an alternator if you do not upgrade it when needed.
If the amplifiers you use are not great, you could probably benefit from a 1 Farad cap. Most decent amps will not benefit as much by using one.
You also need to understand what a line driver is (among many other things).
The line driver will do more for making it sound good than anything else you have listed. Run the line driver as high as possible so you do NOT have to turn the amplifiers up in order to get rated RMS power out of them. You cannot get one watt more by having the amplifier level set higher.
The box design and construction for the 15's will have more impact on the sound than any other thing you have NOT listed.