I've got a 91 F250 with dual tanks and the in-dash switch works so the switch between the tanks must be bad and from what I can tell it must be located on top of the main tank in the back?

Can anyone verify that?

Sure seems like a dumb place to put it but it looks like that is where it is?

 

I changed mine on my 84 f250. It was next to the front tank on the inside of the frame on the drivers side. Took me 2 days to fix. Mostly alone. You'll want someone to be there to help you

 

I believe that it is on the frame on drivers side by the front tank