Need to add Subs to stock Bose

I have a sub box that I want to add to my stock system on my 2002 G20. How can I make this work? Can I split the rear speaker wire to a crossover and feed the subs? Wouldn't that drop the OHMS to 2?

you will need to buy a converter. with this you can split the rear wires and simply plug the rca's also you will need to run amp power and ground and a remote wire.

or just tap into the stock amp :D That's how my amp is set up.