Freightliner Century Cab Flaws, outside air coming in

I would bet that the air is coming in through the bottom door gasket at the floor.
Roll up a couple long beach type towels and put them up against the door bottom and floor and see if the draft goes away. If it does that would tell you if you need to replace gasket or adjust hinges on doors.

 

A few years back I had two friends buy new heavy Alaska spec trucks at the same time, when the weather started cooling off, one of them would not stay warm at all.
Freightliner did not only not put the artic insulation package it was speced for, but somehow did not put any insulation in it, it had to go back to the dealer and stay quite a while to get it taken care of. lol
That is the only time I have ever heard of such a thing.
I know some years western stars have a setting on the heater valve that makes you think someone has left a door off, a guy up here figured a work around for it and fixed all the companies trucks that he was driving for. I was riding shotgun in a blow in one of them, and ask the driver if the truck had a hole in it or what. I had broke down in high winds and minus 30 temps, so was still suited up. he told me maybe my door wasn't closed good, but it was coming from out from under the dash and I told him it would not matter if we took the door off. I told him to turn on the dome light, I was covered in an inch of snow and so was part of his bunk, lol It turned out that it was the heater setting. lol

 

Shakers have as always been known for having the wind blow through them.

 

Mine the wind blows through the speaker in the door 2016 Columbia card board and tape through the winter