I am diabetic. Recently some infection started behind my both ears and it looks like rash. Its also spreading. What is the cure. Thanks.?

Depends on the cause. There are many types and causes of skin rashes. Several types of inflammatory conditions of the skin, allergic reactions, and sometimes bacterial, viral or fungal infections can cause rashes on the skin. Insect bites can also be the cause of a rash on the skin. Have it evaluated by a professional and get the appropriate treatment.
See your doctor. Since you are a diabetic, i suggest your see your doctor about this and do not try to treat this at home. Diabetic patients are more susceptible to certain things, such as infections, that can get out of hand much more easily than in non-diabetics. Please see your doctor.
See doctor. And get appropriate treatment before it gets worse especially since you are diabetic and it is spreading., it could be a bacterial or yeast infection or an allergic reaction, or a bug bite.....See doctor asap. Do not delay.
Rash. Could be a scalp dermatitis or cellulitis. Should be evaluated in person with your physician.