Yes, your testing of the ground side of the circuit proves it good. Moving the switch down a notch used the resistor pathway, which is why the voltage relative to B+ read lower on the meter at that setting.My car was just sitting in the drive way for weeks while I worked on it, not like a kid was playing in the front seat kicking his legs. So those numbers confirmed that the ground the computer uses is good? I still think its odd my radio was acting funny but not anymore.
I mentioned the switch in the A/C control unit earlier, and there have been reports of Kias and Hyundais having overheating issues with that switch, causing melted connectors. But I don't think that would be the issue with yours, because everything always tests fine on the meter at the blower motor connector. So if a connection has separated due to melting, then it should stay separated and not go back and forth. However, I'm not about to say it's impossible that the switch is bad, because you have some REALLY strange stuff going on there. And the radio is in the same ballpark, so any symptoms with that can't be ignored either. And I suppose the fuse box can't be eliminated as a suspect either, however the same argument about the switch applies to the fuse box as well.
One thing I thought of to maybe test out the bad connector idea would be to use a couple of jumper/probe wires to bridge the blower motor pins to the connector slots. If you have small alligator clips to attach to the pins, you could then probe the connector slots, just like with the meter probes. If that 'loose' connectivity makes the motor spin, then the bad connector would be proved. You could also swap the blower motor relay, which would be an easy way to confirm that (either way).