tanks won't switch

Hi, I'm new here and I just picked up a 74 gmc 3500. It has dual tanks and the switch for the tank has been pushed into the dash and I was able to find it but it won't switch tanks. I'm new to the dual tanks and wasn't sure what I should be looking for to trouble shoot it. I was under the truck and wasn't sure if I would be able to jump the tanks but I didn't want to mess anything up.

Usually the selector valve goes out before the switch, left tank is called the main and right tank is the Aux. You can check for voltage at the valve to make sure it is switching properly. You can do a search for wiring diagrams to check the proper wire, im unsure what the colors are right now.

however the switches are lots cheeper then the valve are switches are like 10 bucks at napa and they take just a couple of minutes to install and it sounds like you need one any ways

Well I got under the truck today and the driver side tank is ran direct to the fuel line for the motor. So I'm guessing the passenger tank's pump went out and instead of replacing it they just ran the drivers tank direct.