Persistent yeast in cat's ear?

I have a diabetic cat who has been treated extensively for both ear mites and yeast in his ears by the vet. The vet has exhausted all he has. I'm inclined to think this is yeast as diabetics are more prone to yeast and mites would have been, at least, affected by treatment. This is now to the point where it affects his balance as it is deep within his ears. The best I've had to offer him is to clean out the ears frequently but, of course, I can't get deep within. He is constantly miserable. Does anyone have any suggestions to treat this problem?