YES, they are interchangable!
I was wondering this because my S13 mirror dimmer quit working so I bought an S14 mirror and tried it. The mounting provisions are identical.
The mirror itself is a little taller (top to bottom) a little narrower (left to right) and a little deeper (front to rear), but YES, they do fit PERFECTLY.
I replaced the entire mirror, arm and all. I'm posting this purely for informational purposes, in case anyone was looking for a replacement mirror and wanted to know.
