the way to connect is
front battery to + rear battery , + rear battery to + capacitor, +capacitor to amp
= positive, check whether urs is connected this way
secondly get ur batterys strength checked, i think they r almost gone due to weak chargin,
thirdly the capacitor u are using is not real sound stream, is china copy of soundstream so its not much effective
soundstream doesnt make 3.5 farad
Thirdly ur power cables seem to be weaker than ur application
If u can come to islamabad sometime we can sort this out 