im sure the new cars alternator and battery would be a better test for your system.
do ALOT more research on google about caps and their pros/cons.
there are lots to both sides of the argument.
IMO, to avoid an argument or spending tons more on an upgraded alternator, just add a second deep cycle dry cell battery to your setup. with an isolator of course.
though, the quick n dirty is that if you go above 1000w, you need "something" added to your electrical. Whether its a cap, a battery or a new alternator.
there is some math to be done, taking your alternators output AMPs and getting the # of watts it can produce, subtract your systems wattage needs, subtract a little bit for the A/C and cars electrical. if you are pulling more than the alternator can physically produce, your lights dim.