Sound like they changed the fuel tank selector valve and got the return lines crossed or feed lines crossed. If you have oem switch it will have push on fittins. If aftermarket most likely barbs and clamps. Fuel selector valve is on driver side frame rail (inside) at the front corner of front tank. 2 lines to engine and 2 lines to each tank. Get a light and crawl under there and have a look. You can feel which lines go to each tank. Look at the valve and I think you will be able to figure out which line should be paired together as it switches. I just changed mine so I have the old one still. if you can't figure it out let me know and I'll send a picture of the valve.

 

Caution: If you pull feed lines off they will drain by gravity. Be prepared to push them up to top of frame. I don't think my returns did. I blew a little air through mine as I removed them to stop the flow.

 

I just replaced my valve, if it still has the original lines it is kind of hard to mix up since the lines are pretty much formed in place of where they go due to being that way for so long. The valve is marked but when it is installed you will never be able to read half of it. Here are some pics of my old one if this helps: https://www.flickr.com/photos/moderndefensivemeasuresllc/with/22954195040/