The rear delts usually get neglected, but this is a pretty easy fix and one you can do at home on a daily basis. The band pull-apart will bring your sad little finger-sized rear delts up to snuff quickly.
You don't need a heavy-duty resistance band because you're going to keep the reps high. Rock bottom rep range is 20, done for 4-5 sets daily.
Stimulate new back growth by changing the angle. Use a Landmine device (or place a bar in a corner). Attach a band for a more intense peak contraction.