I drive a 4 door truck and don't use rear speakers. The stock location for my rear speakers is right behind my shoulder, maybe a foot back...it just interferes too much with what's going on in front of me, regardless of how low the volume is.
I've heard/done setups where having a pair of 5.25 or 6.5 midranges(just mids, no tweets) lightly playing the 80-2.5k range works well, adding "fullness" to the music(more specifically vocals). It's just all about the placement, frequency range, and volume at which you play them that makes a big difference.