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

those are about 90% easy to replace, the part that goes into the door is a real b*tch to get in properly but it can be done with some long needlenose pliers. you have to take the door cards off, as well as the "chrome" trim piece on the outside of the door and the scrapers. does the window rattle? i noticed that was the symptom when my window runs were getting towards the end of their life...

__________________
1981 NA 300D 310k miles

Actually, the windows are snug though I think I've isolated the problem. Took a drive around the block and the air seems to be coming from the side mirror. Now the question is how to remove. I imagine I can reseal them from the inside.

Well, for posterity/in case anyone's interested, the issue was the outer rubber seal of the driver's side mirror. The seal wasn't fantastic, and at speed, was creating a hell of a noisy draft. A ring of sealant/gasket maker around the inner ridge, bolted back to the door, voila. Quiet as a 240D can be at 75mph.