Most medical insurance companies have a separate process for mental health coverage.
You might need to start with your G.P. and be given a "referral" to a specialist.
Or, sometimes your insurance booklet will instruct you to call a special phone number which will give you the doctors in your area that are covered by your particular plan.
Seasonal Affective Disorder is also called S.A.D., so be sure to say the whole thing at first when setting up appts...