could i use the positive wire? if i cut it in half? or will it make a differencea battery just for your amp is ideal IF you have a proper alt to supply your reg battery and a battery for your system. you just throw in a battery and use the pos battery terminal to your amp and ground the battery to any location wont help. that battery will be bead in an hour of playing without it being able to re supply the power taken from the amp.
you could run the neg power cable from amp to the original batteries ground or batteries ground location and see if it helps.