I have a 2009 Mazda CX-7.
One headlight assembly has a leak resulting in moisture accumulation in the lens.
If I can dry out the lens, I can use silicon caulk to seal the top part of the lens where I feel the water is coming in from.
Cost of a new assembly is way too high at $220+.
I thought of blowing hot air thru the hole for the high beam lamp.
Hole is only about 1/4 inch which is too small for using a blow dryer or silica gel to dry the lens.
Any one have any other ideas ??
Before going through all that, remove the vent tube from the back of the headlight assembly and make sure it isn’t restricted by something like a spider egg sac.
This is the most common reason why condensation forms inside headlight assemblies.
Tester
If you feel there is water coming into the lens then it must be cracked. If you can dry it out then you can try to seal it where it is cracked but experience tells me that you will end up buying a new one. Do you have the halogen lights or the HID ones? If you have the halogen lights then you can order an aftermarket assembly online and install it yourself for a decent price. If you have the HID lights then I don’t recommend aftermarket replacement. Ouch!!!
The picture you posted is not from a 2009.
Mine is much harder to work on as the air filter housing is in the way. 
Mine does not have a vent tube.
I’m sorry if you misunderstood.
I provided an example of the vent tube found on most headlight assemblies.
You may have to remove the air filter housing in order to see the entire backside of the headlight assembly.
Tester
You might be able to remove the the headlight ass’y and heat it up slowly with a hair dryer on the bench, oriented so the resultant heated air inside (& hopefully the water vapor) goes up and out the vent. I’ve had good luck sealing a tail-light ass’y to the body on my Corolla using silicon caulk.
No problem.
I felt all around the headlight and found no vent tubes.
I may drill a hole in the back being very careful to not hit anything impt.
If that works, I will use caulk around the gasket.
What kind of a headlight bulb goes in a 1/4 inch hole? If you would rather drill a hole in the back of your headlight assembly than remove your air cleaner assembly, I do not predict a successful repair.
I have purchased aftermarket headlight assemblies from Rock auto for my 1999 Honda Civic and 2007 Chrysler Town and Country. They make the vehicles look almost as good as new and they improve the lighting much more than sanding and polishing the old headlights did. And except for OEM models, they are not very expensive.
One started leaking moisture. I dried it with a hair dryer and caulked the area where I thought the leak might be - but that can be very hard to discern without removing the assembly and water testing it. Since it was within the one year warranty, Rock Auto sent me a new one and a label for free return shipping of the leaking one.
Good luck in your efforts to dry and seal it, but I would not hesitate to buy new ones if the problem persists.
I think it is a halogen bulb.
As for drilling a hole to make a vent hole, try to think positively.
I worked for 25 years in a lab and fixed many instruments.
I have a great deal of experience in repairing many things. 
The price at Rock Auto is $239 for the part.
A little high for my budget.
As a halogen bulbs burns at 800 degrees, a little moisture inside is probably no big deal.
It chaps me that Mazda (as well as other auto makers) intentionally makes things difficult for the owner to do their own repairs.
I think that too, (intentionally difficult) every time I pop a hood. But in reality, it is you and I and every other consumer that has made it this way. Imagine if your car was big enough to crawl inside the engine compartment, and access every single part - yes, I have had cars that big! - it would weigh more than my ex mother in law, and get 5 miles per gallon.
Todays cars require trade offs. I mumble about engineers needing to come out in the field and look at the bonehead decisions they made, but then again, I have to work on things much less than the cars of 30-40 years ago.
How do you figure it is the customer that made the engine compartment so crowded ?
When they went to front wheel drive, things got too cramped.
Making the engine compartment one foot longer would have a negligibly impact on gas mileage.
There are advantages to aluminum blocks but one disadvantage is that it contracts and expands.
That caused me to have to replace the gaskets of my oil cooler at a cost of $300.
I had this same problem in my 2007 Highlander’s headlight assembly. I found the advice about using silicone caulking hard to believe, but I did it and it worked. I can even take it to the car wash and no more moisture. It has been over three years since I fixed it, Be sure the moisture is burned off before you seal (of course). The headlights can handle that. I have fixed three cracked fog lights this way on various cars since as well. Worked on all four lights I tried it on. Good luck!
Thanks for the encouraging news.
You have two choices- with no vent, you have to seal it completely with no humid air or water inside OR vent it with a vent tube large enough to promote air exchange and seal it well enough to prevent liquid water ingress.
I got the headlight assembly dried out.
First a drilled a 1/4 hole in the back.
Left car in sun for 2 days.
Moisture evaporated.
Sealed up hole with tape.
Thanks for the update OP. Good to know.
