I've done this on a standard radio boost. Im no expert so I'll leave any helpful advice to Mikey.
As we're on the subject; my boot has a 12V power supply to the left. I got my voltage/wattage meter out at the weekend and it was reading nearly 200w. Would it work if I used that to power the amp/sub? My amp is 400w; 200w is ample for me as I already have the HK system which is very bassey
