When I worked at Hella manufacturing OEM headlights for Ford, Toyota etc, they had vents in the rear housing that looked like small barbed hose fittings with a small U shaped rubber hose pointing down with a small round sponge shoved into the hole to stop debris from entering.
Unless the headlights have a perfect seal against the bodywork, you could seal the lens covers on but I'd have something small like a 1/8" hole or a gap in the rubber seal as low as possible to stop condensation and to let air circulate.