DOG CHEWED WIRING HARNESS BETWEEN CAB AND REAREND NOW CANNOT SWITCH BETWEEN FRONT AND REAR FUEL TANKS NEED WIRING DIAGRAM TO TROUBLE SHOOT

Go to rear pump, disconnect wiring harness and connect power to it directly. If pump runs, you may have bad wiring or the hoses/filter may be clogged. If it doesn't run, you have a bad pump. I

if it is a short then check where the wiring harness comes out of the cab an look where the harness could be worn through

memory serves me the fuel gate valve between the two tanks goes bad ...need to find an old diagram but, testing the switch is done there as well... power to one side switched the gate valve to front..power to the (other pin) switched to the rear tank... find which pin lights with a test light when you call the front tank.. if it lights your valve is bad and not switching

https://s8.postimg.org/caixq1kxh/power_windows.jpg

Click on the link for wiring diagram. Do you have the rear windows locked-out by the master switch, note the diagram.
Is there a wiring harness on hinge side of rear door, probably has a protective rubber boot covering. If problem isn't the lock out switch on the driver door, then we will have to check voltage and ground at wiring harness.

I don't know if this will come thru or not. You may need to get a shop manual to get all of the diagrams to troubleshoot the circuits.
The diagram I found shows two tanks, and each one has a relay for the pump. The relays and gauge are controlled by the PCM and both pumps are grounded together. This may or may not match your truck.

there usually is a dual take valve switch under truck which could be the problem

all you need to do is listen to hear the tank switching valve is working. It will make a soft noise under the rear cab section with the key on with the engine off. zif you do hear it sounding the switch your tank unit for that one tank you try to switch to is a fault. Sorry no illustrated diagrams are on hand, just knowledge

Hi I have a 4x4 2003 chevy 3500 silverado dually with dual tanks
rear tank stoped pumping into front tank and was full . I siphoned the fuel out of rear tank and fuel gauge reads 9/16 full . is there a way to reset the fuel sending units so they can calculate the fuel readings. I always filled the two tanks with the same amount of fuel
until now ? I'm thinking there is a inkine fuel pump

You have duel tanks with a fuel pump in each tank. You also have a fuel pump on the frame rail after the "tank selector". This device only holds a fuel filter that protects the main fuel pump. When you select front or rear tank on the dash, you are selecting which in tank fuel pump will operate. You can check the in tank fuel pump by listening for it to "humm" when the key is turned on..The in tank pumps only feed the main pump and cannot run the engine on their own. Hope this helps

ck wires not sockets sounds like ground problems