3 point belts in the rear were an option..at least I know chargers did. saw a pic of one over at dodgecharger.com

If you peek at your package tray from underneath in the trunk...you can see the slotted holes stamped in it.

I used these to add 3 point belts to the back of my 68..check it out.
http://www.dodgecharger.com/forum/index.php/topic,11621.0.html

and for pics of a factory installed rear shoulder belt....

http://www.dodgecharger.com/forum/index.php/topic,55458.0.html