youll be fine adding a 2nd batt in the trunk, as long as its a sealed batt. also if your just running a 1200w amp, there is no need for a high output alt as long as you have the 2nd batt. i had 2 3000w amps in mine with 3 xs power d3100 batts on the stock alt and had no issues. you can also add a volt meter, i use the stinger one that you can find on ebay for bout 25, so that you can watch your voltage to make sure its not dropping to low.