If you are getting 6Vdc at the inertia Switch, you do not have enough voltage to run the pump. The I.S. is between the relay and the pump, so it should see battery voltage during normal operation. ALso, the pump is only commanded for a short time after a "key on", until the EEC sees the engine is turning over at which time it will command the relay closed to acitivate the pump.
Check your connections at the underhood relay center, and at the power post just behind the battery. You also need to KNOW if the EEC is getting a crank signal (CMP), if not you'll not get fuel, spark, or start.