Non-Healing wound on ear

My cat has had a scab on his ear for 4 months. The vet gave me some topical/steroid treatment to put on it but it isn't going away. She said it isn't skin cancer because of the way it looks. Does anyone have any ideas on what this might be? He is about 17 years old.
Thanks

Has your cat had a senior blood panel run lately? One of the symptoms of diabetes is wounds that heal slowly.