you aren't going to want to go directly from the onboard sound. The line level voltage isn't NEAR enough. You can use a line level booster or something along the lines of an EQ (which will boost the voltage to something respectible). Then from there out to your amps.
Otherwise, you would just use a 1/8" phono to RCA Y. But I wouldn't recomend doing it that way.
As for the home theater thing, don't. Most home audio systems are 8ohm, most car audio systems a 2 - 4 ohm. If you use home audio speakers in the car, you will be shorting yourself out of the power that the amplifiers are capable of (essently halfing the power output because of the 8 ohm config of the speakers). Get car specific equipment. There are SOME speakers out there that some people use that are a crossover from the home audio world to the car (I'm one of those people - Focal Beryllium), however, most of those speakers are VERY high end and are VERY expensive, and they are worth it.
As for your speakers in your car...forget about them. I'd replace them. A lot of stock stereo systems have built in crossovers on the speakers and the speakers are all kinds of funky (like 2 ohm with the amplifier built onto the magnet structure).
Hope this helps!