like i keep saying...rear delts are small muscles. even smaller than your biceps, you don't need to do heavy lifts to hit them. do them on back day and shoulder day. high reps and squeeze every rep on reverse pec/dec. 4 sets.
Agreed with Gimp. Read delts are very small. You must use perfect form and really try to squeeze and feel it in every rep. I see guys doing some heavy ass rear delt isolation exercises all the time and they are hitting their back moreso then anything else.