I had a previous post that my fan blower automatically changes itself all of the sudden, from low, to high.

As I've learned more, the previous post question is no longer relevant, and a bunch of people have already read that post, so it becomes a "dead post".

I've heard it could be the switch, or the blower motor resister.

How can I test either of these to see which is the problem?

1991 300d, 180k