While a fuel pump that has brushes worn to the end of their life can become intermittant like yours and tapping it often gets it going for a short time (seconds to days), wiggling the external wiring would not have such an effect. Common sense will tell you that have you have zeroed in on the problem as external: either a broken wire leading to the pump at the tank, or a connection being moved when you move the harness.

Save yourself a lot of work and connect an ohmmeter to the power leading into the tank, forward of the problem area, and the other side to body ground. Watch the meter as you wiggle the wiring and be very suspicious of the area most sensitive. You may be able to cut open the harness and repair the failing wire easily. Good luck.