What is the treatment for rashes in the neck area?

Vaseline. Babies have "bull necks", and wetness and formula love to get into the creases and cause irritation. Yeast may even move in, making more fun for everyone. A good antifungal like lotrimin (clotrimazole) kills the yeast, an anti inflammatory such as Hydrocortisone helps the raw irritated skin, and a barrier cream such as vaseline protects the skin from future irritation and allows it to heal.
Air dry. Air drying the skin will clear most of these rashes up. If they don't dry up with that, try vaseline to keep the skin soft and as a barrier to keep sweat and milk out. If the rash still does not clear up, see your doctor, this could be an infection and need specific antibiotics.
Keep clean and dry. The neck area can get moist. Rashes in moist areas can have yeast, so a doctor should see the rash to decide whether or not anti-yeast cream or powder is needed. If a neck stays clean and dry, there is less chance of a rash developing.