Map Pockets:
Hopefully no one will simply drop a handgun into a seat-mounted map pocket. If you do you may have trouble finding it in an emergency.
Depending on how it's constructed, cut a piece of cardboard to fit it, and then use that as a pattern to cut out a thin but rigid panel. Metal, wall paneling, plastic, whatever.
Then use adhesive backed Velcro to attach a holster, and inexpensive nylon ones work fine.
Oh, and when you park the car, leaving a gun in the pocket isn't the best of ideas, because it's one of the first places someone will look if they break in while you're gone.
If you are involved in a traffic stop, tell the officer where the gun is, and don't make any moves toward it unless instructed to do so. If they discover the gun on his/her own you may be in for a difficult time...