I did my search and found plenty of help.
I would like to thank everyone who contributes information. It sure helps.
I thought I'd contribute a couple pictures.
My truck ia a 1999 F150 with manual heat/ac (115,000? miles).
My fan was working on high only. With the clues from this forum, I went straight for the resistor. And look what I found...
From the amount of rust, I've apparently been missing a couple speeds for a long time. The melted plug could have been more recent and took out #3.
For a recap:
The resistor is located on the passenger side, under the dash, near the firewall. It's difficult to see.
If you look near the kick panel, you can see the wire harness and the plugs. Follow the wire to the resistor.
The two resistor screws are pointed straight up and mine took an 8mm socket. I removed mine by feel. (unplug it after you remove it, so you can see it better.)