full speed rear wiper?

[GPSnV1]

I survived the ice storm in OKC then drove to eastern TN yesterday. On the last stretch, the rain and fog were heavy enough for cars to pull off the freeway.

I realized that the rear wiper would not stay in motion but would wipe maybe every 30 seconds. It has been so long that I've been in a heavy rain with my 04 X5 4.4i that I didn't remember what to do.

When I reached my destination, I looked at the manual (p.67) but it leaves me confused about how to keep the rear wiper on motion unless I am moving in reverse.

So what am I forgetting?