I don't run rear drivers at all. but if i did, it would be like this (copy and paste the link): http://www.carsound.com/UBB/ultimatebb.php?ubb=get_topic;f=5;t=001818
personally, if I used rear fill, I'd run them in mono, with a narrow bandpass filter. that keeps the frequency range limited and prevents ruin of the front stage imaging.
i've never tried it. but it seems like if you did it and used midbass drivers with the narrow 2way filter like glass suggests, the result would be a delayed effect that succeeds the front image without upper frequencies. kind of giving a true fill maintaining the forward imaging. does that sound about right?
no delay.. but otherwise yeah you just get more midrange from the back to fill in.. without the highs to draw the soundstage back into the cabin of the car