Rbm, thanks. Yes, I definitely have some ad hoc wiring. I unwrapped the electrical tape around the wires between the tank and the black connectors. In doing so, the connection to the four prong plug that exits the tank came apart, revealing four separate black slide-on connectors, kind of like a crimp-on connector, but not crimped. Connections weren't very clean, and evidently were held in place only by the tape wrap. The fuel pump has been working, so at least some of the connections were good. At this point I removed the tank, since I could see floating debris in the gas, and I wanted to clean it out and replace the fuel filter anyway.
I removed the large nut holding the sensor onto the tank, and pulled out a plastic covered cylinder, which I assume is the sensor, but it doesn't have a float like the others I've seen. Maybe this is the original version that was changed on later models? Two wires enter the unit from the tank, one purple, one gray, which must come from the fuel pump. There is an opening toward the bottom of the cylinder where you can see what wires enter the four prong plug. There are five wires: two gray, one purple, one reddish orange, and one dark brown. The two gray wires look like they join to one of the four prongs. I've attached some pictures, to give a general idea of the layout. Perhaps I could try to run the test you suggested on the prongs in the plug, though even those don't seem to jibe with the standard color wires. Is that a test I can run with the tank off and no power to the circuit?
The only white four prong connector like the one in your picture that I have found is in the relay box, and is not attached to a plug. The wires going into it are brown, green/purple, red/yellow, and green/brown. Don't know what the function of this would be.
Thanks again.