the
lower the temperature of the air, the less moisture it can hold.
Thus the term relative humidity, eg 60% at 20 degrees. might be 70% at 15 degrees. (don't quote that as its just an example and I wont be looking up the actual figures)
Therefore when the tmperatures drop, or you wash the car the temperature of the plastic/glass is cold, which reduces the air temperature to a point where condensation can occur. and so is common this time of year.
(this is why we get condensation on glass in the winter.. colder temps, cold glass, we breath out moisture, we wrap up warm... we put the blowers on leaving a humid high temperature in the car which we then leave to cool resulting in condensation)
turn heaters off towards end of journey and open windows to provide an air change sometimes helps in the morning after. but forget it if you have anything wet on the car, like a coat or the rap you wiped the windscreen with yesterday..
headlight wise: clearly other aspects impact this:
any leaks in the headlight can allow water inside.
how well is the headlight unit sealed? - or what were the conditions like when it was last open?
(if you say headlights are sealed, - how do you change the bulb?)
much like a bathroom increasing the temperature, or introducing air replacement (extractor) is the way to correct.
In the case of headlights, turning them on increases air temperature, which can then appear to remove the condensation. - however when the air temperature drops it can re-form.
Alternative options are:
remove back of light and leave to clear. then replace. - hoping that the relative air humidity is low at the time as this will
help.
or open rear of headlight and use a hair dryer. - but if you do this leave the back off for a while to cool, or use cold function on dryer for a while before resealing to remove the warn humid air.