what is the purpose of a Car Audio Capacitor?

[yumyumchef]

i have an mtx thunder 1501d monoblock amp that puts out 1500 watts at 1 ohm continuous. it is powering 2 12" pioneer subs. the subs are pioneer's flagship subs and are rated at 3k watts peak and 1500 continuous. the stereo is in conjunction with the factory interior stereo in my audi a4 2.0t and sounds amazing. what brand of cap should i buy? how many farad capacitor should i buy? please teach me!!! thanks