Steve-UK,
Thanks for the information - I think you're totally on to the root cause. I'm still am not exactly sure which side of the small triangular rubber seal is the issue? I believe it's the point towards the hood in the front of the mirror. To further clarify the rubber seal tracks the body about even with the front of the mirror (mirror side) and then runs clear to the back of the mirror towards the front of the car. The problem area of seal is that area that is actually over the vertical channel (back side of mirror closer to front of car)? This is the area that needs to be sealed?

A picture would be worth a thousand words if you could post something to get us by until a video is available?

I was all set to POR 15 this today per Andrew's game-plan but I'm just a deer staring into a headlight without fully understanding root cause.

My plan from here:
1) I'm going to see what dealer will do. If long process without an answer I'm moving forward with POR-15 (I'm going to make sure channel is dry with hairdryer as first step and use all the Por-15 recommended prep chemicals to prepare surface)
2) I'll seal triangular part of rubber seal once further clarified per above.
3) Also seal the two channel opening for good measure on track but I don't believe this is the issue.

Concerns:
1) I'm not sure we will win the war once rust has started no matter how small. For this reason I want to hear what dealer will do. Also I don't think we should have to repair especially if still under warranty - but I'm also concerned about Car-Fax event (hence deer staring in headlight).
2) I'm not convinced you can put Por-15 over a painted surface but it still seems the best Do-it-your-self solution.

Thanks for educational information on this post.