Shoulder external rotation in most people is like a good sense of humour in old white men, non existent.
Often I find that people are trying to do overhead lifts or other exercises but they feel really restricted in their movements and don't know why. When your shoulders lose range of motions, its usually because they have lost strength in certain muscles around the shoulder joint. The most common muscles that show signs of weakness are the serratus anterior, the lower trapezius and the external rotators. When doing work on your external shoulder rotation, keep in mind that if you are not feeling it in the back of the shoulder but rather the biceps or upper trapezius, then you are doing it incorrectly or you have chosen one that is too difficult for you. If you are unable to gain anymore range from working on external rotation alone then try look at some of the other muscles around the shoulder joint that I mentioned.
A few pointers when trying to increase external rotation:
Lower level Unit 4, 175 Oxford Street, Bondi Junction.