installing cap fail?

Hello i just recently got a used 2008 lancer gts, before the lancer i had a 2001 ford sport trac witch i had a 12" 500w [1000wpeak]sub kicker with a 1000 watt koiiler 4 chanel amp everything on

4 gauge wire [no capacitor] i sold the truck but removed my sound system sub,wires,amp

now i got the lancer and i installed my old sound system the car has a built in sub and amp so i added my set up as well the sub did not perform well and my head lights stared to dim a bit.

so i got a cap the boss CPBK2 2 farad and i installed it i charged it correctly and grounded well. i test it with a multi meter for the ground.

when i put everything on its place for the test the capacitor started to do a constant beep when the bass hits. indicating one of 3 things

Audible warning tone for reverse polarity

Audible warning tone for low battery voltage

Audible warning tone for voltage overload

the manual does help at all

things i did to try to fix it

sanded off the ground area for better connection-nothing same beep

unpluggedand trun off the amp and let the cap run by it self to see if the amp was the problem-same problem so its not the amp

i did built for my friend 6 months back 2-12" jbl 500rms[1000peak]" and a boss 2000watt amp and 8 gauge wire and the same cap boss 2 farad

so i got the one in the lancer and test it on my friends built to see if it was a bad cap-the cap was full operational no issues

with everything i verified if there was a voltage drop with the multi meter on the cap the amp and the car battery everything was normal

i went to walt-mart and got a Scosche 500K Micro-Farad Capacitor cap] just to see if the cap was 'too' big

similar problem the cap will go off after a few bass hits the LCD digital voltage display will turn off [still don't know what that means so i take it as a the same problem]

running out of options on now

what i have left is lower the gauge wire from 4 to 8

directly install the positive to the alternator

get a extra battery and ditch the caps

make new ground connections holes on the car

anyone can help?

sorry English is not my main language sorry for bad English