Whether it’s a 379, 389 or anything similar that has west coast mirrors mounted to the cab (and not the door), this down and dirty method and has been done 1000s of time with success.
Follow along with the pics and if you need further instructions, drop me a line. I left a lot of slack in the coax for emphasis, but it will snug up well. In the cab the only part you are going to see is the 2” going by the air pull, but you will never notice it, unless you have issues.
Now you can be the anal fuk that has to run the coax through the mirror bracket, through the door, up the A pillar, drop the headliner, and do as factory do. If you don’t mind a 4 hour job, cut knuckles, broken plastic tabs, a sag here or a sag there, then have at it.
This method will take you 14 minutes and keep in mind, you do have to replace coax, especially that factory shit. And after the first time, it might take you 7 minutes to do. The hardest part is notching out the aluminum rib that holds the weather seal in place. Cut a 1/2” wide tab with a cutoff wheel, channel locks to grab the tab, then feed it through.