Your system should ideally be set up something like this:
First things first, think of the active sub as another amplifier. Therefore, whatever you give your amp currently powering your components, you have to give to the sub.
So, unless you went OTT with your first power cable it's unlikely that it's going to be up to the job of supplying your amp and your active sub.
Get yourself a new amplifier wiring kit, with a power cable capable of supplying both amps (check wattage).
Replace your existing power cable with the new one from your wiring kit. At the end of this cable connect your power distribution box.
You can use your old power cable to then take a positive supply from the distribution block to each amplifier.
Run a second set of RCA's from your head unit to the location of the amplifier. Ifyour head unit does not have more than one set of RCA outputs you'll need to get some rca splitters (any half-decent audio shop will have them).
Daisy chain the 'remote on' cable from your current amp to the location of the sub.
Give you sub box an earth (if you're struggloing for earth points, buy an earth distribution block as well and use the same principle as you did for the power, ie keep one point and split it between the two pieces of equipment).
Seriously, this is easy stuff man, you won't have a problem. Any Q's, give us a shout.
