Was barreling down the freeway last night and noticed a draft coming from the driver's side window. Ran my hand up and down the frame, definitely not my imagination.

Thought maybe it was the door seal, so checked it out this morning.
Brand new. After a little digging, I found the window seal (window run) pretty cheap on eBay -how difficult of a repair is this?

w123 window run | eBay

Is this a reasonable place to look for an air leak or am I missing something obvious?

EDIT: just read that rear mirrors are often culprits in other cars but couldn't find a specific reference to w123s