Stereo is just 2-channels recorded in a way to recreate spacial sound (like you wrote). All car stereos simply split the stereo channel and send a copy of the signal to the rear speakers to "fill out" the sound and also provide music for passangers. Some head units might do some sort of "virtual surround sound" with the rears, but it is just guess work on their part.

Running just one pair of RCAs back to a 4 channel amp does the same thing as running 2 sets of RCAs except that adjusting the "fade" (relative volume between front and rear) has to be with the gain controls on the amp instead of using the "fade" option on the headunit/empeg.