Hugh - I would drill a 1/8" hole at the bottom-must part of the headlight shell.

That way the moisture might drain out or the heat of the bulbs might drive the moisture out.

You hight add a smail flap to make this opening one way so that the cooling of the headlight case won't draw moisture in on a damp day.

You could easily make a flap by using a piece of thin plastic and tape it on three sides.

In Emergency Medical Technician training this is called an occlusive dressing - used for a chest puncture to allow air to exhaust from the puncture and stop air from being drawn in, to avoid collapsing a lung.

EMT use baggies for this flap.

If my idea does not work, it can be easily plugged up.