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. 

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.