Cat is scratching behind ears until she pees and vet doesn't think it's ear mites - what could it be

what did the vet say it is?

She didn't know - kitty Prozac Isn't working

so other possibilities have been ruled out -- such as adverse food reactions, atopic dermatitis, and parasitic hypersensitivity?

Abnormal repetitive behaviors in cats are derived from normal behaviors such as stalking, chasing, grooming, etc. These may be exacerbated by stress or anxiety such as alterations in relationships with people or other cats, or may be inadvertently aggravated by the owners either reinforcing or punishing the behavior (increasing conflict and anxiety). If these behaviors occur out of context or in a frequency or duration in excess of that needed to accomplish the task, a diagnosis of compulsive disorder should be considered. Medical problems must be excluded, because they can be responsible for many of the same signs. For example, self-mutilation, excessive grooming, and/or self-directed aggression can be due to any condition that might cause neuropathic pain or pruritus such as adverse food reactions, atopic dermatitis, and parasitic hypersensitivity.

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 Quoted from:  http://www.merckmanuals.com/vet/beh...stic_animals/behavioral_problems_of_cats.html

If she is that itchy, she needs the pruritis (itchiness) treated, not only behavioral modification. Anti-inflammatory treatment with corticosteroids is first treatment. Then, discerning why she's itchy. If she had a single flea bite her, then die or she swallowed it during grooming, its saliva can make her that irritated and itchy all over if she has a sensitivity to their saliva.

Scratching the neck and ears is the most accessible region with her hind feet. Also, excessively licking at the tailhead, over the rump, is common with flea allergy dermatitis (FAD).

Prozac takes at least 4 weeks to be effective, if this is truly a behavior due to anxiety induced compulsion. Different cats respond to different anxiolytics. After 4 weeks, if not effective another is tried.

Pruritis can also be due to bacterial infections, atopy (skin allergy), topical irritant exposure, shampoo left on skin causing dryness, low dietary omega 3 fatty acids, etc. Prozac won't help these causes.

What did your vet rule out during the exam? Any diagnostics done? Describe your cat's lifestyle, meds (including preventatives/supplements), and exposure to other cats, outdoors, stress, etc.