Is a Misted Headlight an Mot Failure?

It'll only fail if it distorts the beam. Will fail if it affects headlight beam pattern, Or reduces light output.

How do you get moisture out of a headlight without opening it?

Use a hairdryer to dry out the moist air without breaking the seal. Use a hairdryer to blow hot, dry air into the vent or on the outside of the headlight unit. As it heats up, it should dry out. Wait for your headlight unit to cool off to determine whether this strategy was effective.

How do you get moisture out of a headlight?

If the condensation is minor, then a bit of compressed air or a couple of desiccant packets can get rid of any moisture in the housing. If there is a lot of condensation, then you can use a hair dryer or even an oven to dry out the headlight housing.