I have a 1987 six cylinder automatic truck. I'm trying to get it to run on the rear tank. I put the tank selector on the rear indicater but it still pumps from the center tank. In order to test the pump, I unhooked the center tank pump wiring and hooked it up to the rear tank pump. The rear tank is removed; but still the pump won't work. The truck still runs without both tank fuel pumps hooked up. There is a pump on the frame: what is that?
The truck will still run and idle on that frame pump at the same time the 2 pumps inside the fuel tank are unhooked.

 

You have low pressure pumps in tank and a high pressure booster pump on frame. The high pressure pump is sucking fuel through the intank pumps and working hard at it to get fuel to injectors. You may be getting pressure but volume is limited.
Figure out if wiring is bad to the pumps or just if both pumps are bad.