Power cap
 

I'm currently running two caps and was wondering if they are doing anything constructive, I'm running 3 amps, front components, rear components, 6x9s and a set of components in the boot

Why the need for so many speakers and no sub? Power caps do work well but are mainly for a big amp running subwoofers when the bass drops and the amp needs power instantly. Saves it dragging the power all the way from the battery and dinning your lights etc. it gets the power is needs from the cap first.

Couldn't tell you unless you wanted to post up what size amps your running, and what draw they are getting from the speakers, why have you got 2 caps? And what size are they?

Also power caps won't cure the "dimming lights" it may help towards it in some respects, but far from a cure, A cap does it's best to keep the voltage at a certain level, but if the voltage changes so will the cap voltage. The cap just makes the change smoother. The dimming light syndrome isn't a voltage issue but rather a current (amperage) drop. The voltage simply follows what the current does. A cap does not help this situation at all. A cap doesn't provide more power nor does it try to correct it.

Sorry forgot to mention 2 12" subs and I adopted the set up