I've heard that caps dont do anything. Ive talked to my audio guy countless times about this and he says he never notices a difference with a cap. I would try groundwires or optima.
you need a new audio guy asap! You don't "notice a difference" because it doesn't do anything for sound. It's designed to reduce wear on your alternator and battery. Sounds like he never took a physics class.
Yeah polykarb, probably a system issue, but the odd thing is that it's only on the left side. It is possible that the MTECHs are drawing too much current and the wiring to the lights is starting to go. Between the system and the MTECHs it might be to much strain on the electrical system if you don't have a cap.