When i have my music up loud, my lights dim when the bass kicks in - looks pretty silly! To be honest it's not that noticable but i really notice it now because i keep looking out for it.

Is there a cheap fix? Or will i have to get a power cap or new battery??

I only have 1 12 incher!

Thanks.

Check your earth is good from the amp!! and that the earth lead isnt to long a shorter earth lead is a lot better.
Your only running a 12" sub this shouldnt happen.
One other thing if your amp isnt a high enough wattage to cope with the sub you could be kaining the tits of your amp!! hence the dimming.
If your amp is a goooden then the only other way is like you mentioned Power Cap!!.

No, the amp matches the sub power-wise. I will check the earth.

Thanks for the tip.