For rear delts try something like reverse fly, but you have to light weight and not go too far back else you will be working more of the back instead of the rear delt.
What do you mean too far back? "Above" your back or letting your arms go back towards your waist? Interested if you have a good guide on form.
Sorry i do them using a cable crossover type machine, so like a chest fly but in reverse, i find if take my arms too far back and hands will end up behind my shoulder blades, it works the back more.
Does this make sense?