How should I care for behind ear incisions after face and neck lift? They feel itchy and irritated.

The post-op instructions should be always given by your operating Surgeon. Without pics- it is impossible to say. The quality of the neck lift is directly related to the position and length of the incision behind the ear. Often, when a large amount of skin from this area is removed - the neck will look much improved. 

Your surgeon will give you detailed instructions on the specifics
of caring for your face and neck lift incisions as part of your post-op
recovery. Some rules that you should be sure to implement, still, are keeping
the incisions regularly clean, avoid direct exposure to sunlight and frequently
applying a sunblock with an SPF of at least 30 before going outdoors, and
incorporating foods high in protein and anti-oxidants. Try your best to be as
patient as possible with your healing incisions and be sure to schedule regular
visits with your surgeon to ensure that your recovery is proceeding as planned. 

Peroxide is good to wash off dried blood but by now there should be none and you may be just irritating your wound.  Keep them clean and dry.  Polysporin can be good initially but if used excessively can also create an irritation.

Specific one instructions should come from your own surgeon. In general incision should be kept clean and lubricate. Avoid rubbing are scratching

Facelift incision care differs among surgeons. If this itching is tremendously bothersome then ask your surgeon to recommend or prescribe something to combat this. 

Consult your surgeon first but I think you should definitely stop the peroxide and topical antibiotic. It's likely the cause of the problems.  It's best if you don't put anything on it. I've seen plenty of patients run into this problem when treating the healing incisions with peroxide and topical antibiotic. I never have my patients apply peroxide and I only use topical antibiotic for a few days max. 

First, you should really follow your own surgeon's instructions. Sometimes there can be a little delay in the healing behind the ears in the crease. With that aside I like Aquaphor as a good ointment that promotes healing.

You should ask your surgeon what they would like you to use.  Itching is normal as the nerve sensation returns to the area.  I have people use ointment mostly to keep it moist and to help with any scab formation.    

Postoperative care instructions will vary from surgeon to
surgeon, but I would not advise the use of peroxide for your behind-the-ear
incisions. The main goal of incision care is to keep the incision clean.
Although you may think it's keeping the wound clean, peroxide can kill healthy
cells and actually impede healing. It can also be harsh and abrasive and
exacerbate the normal amount of itching that's expected as incisions heal. You
may want to discontinue use of peroxide and see if the situation improves. If
not, it's time to visit your surgeon and find out what's going on. Persistent
itching could be a sign of a yeast infection or other issue within the wound.

It is normal to have itchness around the scars as they heal. Using antibiotic cream may help but make sue to follow the instructions of your PS. I wouldnt be using peroxide at this stage but again you should see your PS for reassurance.