Would it be better just to get some deepcycle battery and put it in the trunk rather then geting a cap.?

 

A deep cycle battery will help you play your system with the engine not running longer. A cap is used with a running engine's electrical system to smooth of peaks in heavy power draw when the bass hits(mere milisec's worth).

If you are have very slight dimmage, the cap will help. If however you're having dimmage that you can obviously notice... then a high output alternator will have to be your only choice... oooooor step down to a smaller power system in which your stock electrical system can provide power for...