You've got a bad connection somewhere. Either the pin connecting the two halfs of the firewall connector are bad, mislocated, etc., or you've got a resistive connecting somewhere else in the system. Try pulling all the fuses and see if your voltage magically reappears at the fuses. I bad connection will present resistance to the power circuit, and if you attempt to pull current through it, your voltage will drop to zero volts. Remove the fuses which disconnects the loads, and the voltage reappears.
When you measure voltage at the spade connector, did you use a voltmeter? If so, try a test lamp instead. The test lamp will pull a little current through the circuit. You may see no voltage again when the spade is loaded with the test lamp. If that looks good, focus on the connection at the firewall and beyond.
Are the pins in the two connector halfs lined up? Check voltage on the fuse clips at each end of the fuses. Sometimes, the corrosion on the clips can be enough not to power the fuse. In other words, with the fuse installed, you may see one clip is hot, but the other clip, and possibly the fuse are at zero volts.
