The best thing to do is get a speaker level to line out converter. Put that between amp and the stock head unit.
jaak was kind enough to post the wire outs for a non Bose unit HERE .
I do not advise splicing a sub amp into your rear door speakers...unless you just can't stand to do it right. Mine is leased so it will be easy to remove the sub amp without patching up the factory wiring. What happens if you splice into the door speakers= the impedance changes enough that the stock amp freaks at about 17 and starts to "crackle", I am thinking if you like to crank ur tunes, you will smoke your stock amp splicing like I did. I get humongus bass, but not much mid to tweet comparatively.
If you want to do it right:
Get a line converter with 4 outputs--2 for an amp with 4 speaker level outputs to the doors, and 2 for the sub amp.
Put in an amp for the door speakers that is stable to 2 ohms and an amp for a rear sub. Get a band pass cabnet or a sealed cabnet, your choice, and put it in the back. Most amps bolt nicely to the side of sub enclosures. Generator did a very nicce setup---> HERE