Download this. You'll need it. That's the 1973 Dodge service manual. I know. You have a Duster, but the latest Duster manual there is for 72 and I believe 73 is different. The 73 Dodge should be very close. At least this way, you'll have the wiring diagram, because you'll need it.
The first thing I would do is look for ground wires on the light circuit "back there" and see if they are dirty, loose, corroded or whatnot. Do the turn signals work in the front? If so, then you can likely isolate the problem to the rear......maybe. lol
http://www.mymopar.com/downloads/servicemanuals/1973_Dodge_Chassis_Service_Manual.zip