1989 Ford Aerostar fuel pump-checks

When I turn the key on the fuel pump kicks on some of the time, now it doesnt kick on at all, I've replaced fuel relay, replaced fuel pump, checked the connector at the inertia swich with a test light, (light turns on for a couple seconds when the ignition is fisrt turned on, and stays lit when cranking) so does that mean the inertia switch is bad? I just cant figure out why the pump doesn't turn on, I re-checked the pump twice and wiring, every thing is connected right in the tank. At a loss, can you help?