My 05 Lariat has cold air coming into the passenger footwell from up under the dash. Wife alerted me to this but didn't really listen until a buddy mentioned it while we were driving in minus 30 celsius weather with full heat on. Can't seem to get it to stop. Doesn't matter if heating system is on or off. Anyone had this issue? Starting to get cold here again and we've got a few long distance trips planned.

 

I have the same problem but have not investigated it any further.
Just another straw.

 

If the driver window is cracked open, the passenger's feet may get cold. That's the way the air flows. I'd bet that this is your problem. Are you a smoker? Smokers leave the window cracked open.

A bright light will be the best way to find any air leaks. Even if you can't see the hole, you may still see the light shining through. Have someone shine it from inside the passenger compartment, and the engine compartment. Have them move the light around at different angles.
If light and air can get through, so will water.
You had better take off the panel covering the fusebox and shine a flashlight down in there, looking for water. This is where it accumulates if the windshield is leaking.

 

Could also be the blend door actuator needs replacing, although sometimes they can be recalibrated by disconnecting the battery for a few, then reconnecting. Easy enough to try with nothing to lose.

 

The cold air enters the cab from the right side so i don't think it is blend door issue.