forget the cap.
get a better ground.
at least do the ground from the battery to the chassis.

 

In a DC Circuit, (Car Battery to Amp) it acts as a battery. In AC its a filter. This is DC application not AC. you are not going from the ALT to the amp. than ks anyways

 

the alternator creates an ac feedback that does get filtered by the battery but there is still a residue of it and is harmful to SQ

anyway do your homework on the ac ripple from the alternator and come back and talk i shitting you lol even better grab a osciloscope and do the test yourself with a cap and without

 

even better ill stop it right here and tell you what you want to hear. since your running 500watts rms amp yes a .5farad will be just fine as of recomendation i dont know just make sure the ESR is very very low to be affective and check if it was tested at a lower voltage than 20volts cause they say its .5 tested at 20volts. soooo what is it at 14.4 volts??? .1 maybe just check anyway good luck