I have cured my Siamese cat and 2 cats I fostered of ear mites using a mixture of a boric acid solution and ACV which I placed in a dropper bottle. I believe the amount of boric acid I used was 1/4 tsp. in 1/4 C. filtered water. I usually put the water in a heat resistant glass measuring cup and bring it to a boil in the microwave then add the boric acid and stir or put back in the microwave for several more seconds until dissolved. I then allow it to cool (or if in a hurry place the cup in the freezer for 5-7 min.) and add about 1 tbsp. of ACV.
I first clean the cat's ears with a cotton ball with coconut oil (makes it easier to remove the dirt) and then moisten a couple of cotton balls with the solution I prepared and press it on each of the cat's ears so some of it goes in the ear. The cat will shake but this mixture works fast, relief of the itching is almost immediate. It must be applied at least 2x/day, usually only 3 days are necessary but I believe the first time I did it for a whole week. Anyway, when the ears remain clear and the cat's not scratching anymore you'll know he's cured.
NOTE: Borax can be used in place of boric acid, I think it's more stable, boric acid tends to lose potency after a couple of days.