I believe the manual says it's standard equipment on all X5 35d models.

There are quite a few features that are not advertised, that one needs to read the manual in order to discover. It's very extensive and informative, if not a bit awkwardly written.

Nonetheless, all mirrors are heated, and there's no notation next to the reference to suggest it's an option, as there is next to other optional items.

I'm sure others will chime in, but I'm 99.9% sure it's standard on all. I'll look at the manual when I get a chance or you can find it here on the website in PDF form.