Maybe for cars up in Canada, but you don't see it very much in Coupes or Fastbacks in the US. I added one to my 68 Coupe, doesn't work very well. Not a really rare item, but not very common option.
In 1968 4.1% of coupe and 8.7% of U.S. Barracuda production got rear air defogger (no conv's of course). Right side mirror is 3.0% overall.
I would not expect the 69 numbers to be a whole lot different.