I found the fuse by removing the top plastic grille and the smaller metal grille on the right behind it. The fuse was blown, so I replaced it. Closed everything up, plugged it in, put a hot dog in and started it up. It worked for a second and then quit again.

So what would be your suggestion as to what to check next? I see door switches mentioned often as a cause of blown fuses in microwaves. I've looked at the videos here, and I think I can handle replacing the switches. But how do I test them, to see if that's the real problem?
Thanks for your help.