Truck won't start even after replacing nearly everything!

My 2011 ford Ranger started having rare episodes of not cranking, just rapid clicking noises. The battery is charged. It occurred rarely and would crank/start usually after a few minutes. The times it happened it was raining (but may have been coincidence - lots of rain this year). It finally stopped altogether at a gas station. I got it jumped off, drove it home, and it never started again (even with jumping). Just rapid clicks. I already did all this: checked battery and cleaned cables, checked the starter relay (ok), replaced the solenoid and starter (may not have been the problem), replaced the ignition switch (probably wasn't the problem). It still does the same thing! I'm reading 12 volts everywhere. If I take the starter off and connect it directly to the battery, it works fine. What else could it Be? Can the igniton (key) itself cause this?