i have a 1998 eclipse spider the fuel pump runs out of the tank but when i hook it up to the wiring harness it doesn't. i have 12 v at the harness ???

Your pump will not come on with just the key in the on position. These do not prime in the on position. They will only come on if the engine is actually turning over. Test both wires going to the pump as someone is actually trying to start the car. You should get the ground circuit then on the other wire. If not it could be a crank position sensor or ecu issue. Also check your negative battery cable connections on the frame and motor. These cars do all kinds of weird stuff when they do not have a good clean and tight frame and motor ground connection.

totally agree most faults are through poor earth conections