Hi, I'm considering hooking up a subwoofer amplifier to an '06 Honda Civic. If I purchase an amplifier with speaker-level inputs, could I just unplug the wire from the rear speakers, and basically make a Y for each positive and negative connector, running one end to the speaker, and another to the speaker-level input on the amplifier?

My overall goal is to leave the factory amp powering the speakers, and just feed off the speaker signals to power a subwoofer. My concern with the above method is that the sub amplifier would take too much power, and the rear speakers would only be half as loud as the front speakers. I know more than the average person about electronics, so technical explanations in terms of resistance/current are acceptable, although I do not want to have to make my own circuits for the project.