it can be done. that is how i had the setup on my first system.
you will need to go to radioshack and buy a rca converter.
when you have this, just tap into one of the speaker wires. after connecting the rca converter, you can use that signal to hook up to u'r amp.
(When using RCA inputs you do not use an "OHM" input anyway, it is using "VOLTS" and the higher the volt output ie. 2volts, 4volts and now they even have 8volts, the more effective the signal is to your amp, meaning a much cleaner sound with very little noise, more or less). It
No, the rca convertor DOES NOT change the Ohm Load on the stereo. It is actually called a "Hi/Low level input mod" and all it does is simply connect to your right speaker(+&-) and your left speaker(+&-) and then converts it into an rac female out put R & L so that you can connect an amp to your factory set-up. Usually you can find one with a gain, for a little added boost, but overall that's all it does.