I think the face/neck pulls are one of the best suggestions. Beyond those however, I think you are hitting the posterior delts to a decent extent with compound movements. Therefore, I would hit a few isolation exercises for direct stress. After all, we are body builders, not necessarily athletes. So, some isolation is very helpful.
If you have access to a pec-deck/machine flye, I would hit some reverse flyes on there. If you don’t feel your rear delts screaming then you have a serious “trigger” in place. By trigger I mean that other muscles have compensated for the lack of rear delt strength and/or activation. For instance, those that always use lots of bicep/grip when hitting pulls tend to get very little lat work done. The biceps/forearms are triggered as the primary mover instead of the lats. In this case, the trainee needs to unwrap the thumb, only gripping the handle enough to keep the bar from slipping, and mentally focus on lat activation.
In your case, to get rid of this trigger you might use a few isolation movements like the reverse flye. Mentally focus on recruiting the rear delts, not the medial heads nor rhomboids/traps. Those muscles will contract to stabilize the shoulder, but should not be the primary mover. Keep the shoulders neutral, don’t let them move to and fro as your perform the flye.
When performing face/neck pulls, bent laterals, or reverse machine flyes, make absolutely certain that your elbows ALWAYS lead the movement. If your elbows are not totally pulling parallel to the line of force (the cable/dumbbell, wrist, elbow and shoulder joint should all be lined up in the same plane) then you will lose the activation of the rear delt.