ohh bhaijan u dont have to install capacitor in ur setup
just go for proper wiring and put a condenser will cost u around 200/- it will eliminate ur distortions..
coil has nothing to do with ur sound system ... u have to put one good quality wire from the the positive terminal of ur car battery to the amplifier ...
dont mind but from ur comments it seems that u r not at all technical .. just meet any good system installer he will sort out all ur issues... nothing to worry about ..
and the picture u copied above is of te guy who installed 4 woofers in his car which need power stability to get maximum output... capicitor used to store current and provide uniform ampares to ur amplifier.. thats it...