Could you expand on this? What do you mean by the "existing speaker lines"?
So simply unplug those (carefully, use tweezers to squeeze the connectors and they'll pop off without friction), and connect those on some cables which you'll run to your speakers. The connectors can be found at your local electronics store.
The speakers you'll be connecting need to have a crossover that you can bypass (typically four connectors on the back versus two, with a piece of metal in between to short them). Total upgrade would be to mount all the electronics inside, but I haven't dared to cut mine open yet