The title may sound a little yucky, but I've found that more than a few people suffer from a painful and/or bleeding crusty rash behind the ears. It looks like a kind of eczema or psoriasis. This seems to be more common in children perhaps. It's actually easy to get rid of...at least this worked for us!
Clean the area at least twice a day with a warm wash cloth. Don't rub hard, if you do you could cause the rash to bleed and be very painful. Just let the warm water soak it clean.
Let it dry completely. This is an important step! You may either let it air dry or blot it dry with a clean cloth. Moisture will encourage bacteria growth.
Apply Cortizone 10 to the infected area. This is what we used, but Cortaid or something similar could work just as well. We applied it twice a day, but you can do it up to 3 or 4 times daily. Continue until rash is healed.