My old Ford MACH MP3 didn't a have subwoofer output, so my Bazooka took the signal off the rear speakers. Just patched into the existing cables. Since the Bazooka is self amplified, it didn't suck all the juice out of the rear speakers.
The new Alpine has sub out, so now it feeds the Bazooka directly.
If you're connecting to the stock system, then you'll need an amp with "speaker level" inputs, since the factory amps don't have a "pre amp" output.