Here is a copy from the 87 Factory Wiring Manual , It shows the wire you are talking about (circuit 490) is a switching wire back to the relay and then conects to the "ECM" (engine computer).
I will include the wiring to the "Tank Switching Valve" which show the circuit back to the sender/Fuel Pump
I know these diagrams can be hard to follow , take your time and follow the circuit number.
Hope this helps you out...
Desert
