Birkins have, or can have, heated windshields. Tom can fix you up on that. I bought a new one to replace a cracked one earlier this year. I would expect your Birkin has a heated windshield.
Of course, I'm going to use the old frame for a lightweight, plexiglas windshield. I just haven't decided if I'm going to cut it down or not.
The heater core came in a box that was with the car, but is installed again for fall rides.