Best bet is going to be an aftermarket amplifier to power a sub. Those factory amps just don't push enough juice to power a sub, they weren't designed for it. Plus if you wanted to do that you'd need two subs since the amplifiers are not capable of running mixed mono. Wattage is probably somewhere in the neighborhood of 20-50W (I'm guessing), you could try
www.alldatadiy.com for more detailed specs. Might also be in the owners manual.