Not sure which plug you are saying wasn't connected, but I have seen a lot of trucks that came factory with the heater element in the separator having been replaced with a part that did not have a heater installed, so the plug just did not have anything to connect to.
The water in fuel sensor is kind of tubular, with the contacts about 1/4 away from the aluminum housing itself. Lots of times algae grows inside there and even after you drain the separator, that gunk is still on the sensor and the light will continue to say that there is water in there, when there isn't.
Did you try to reconnect that plug? I am curious if your water in fuel light comes on when you plug it up. Thinking the previous owner disconnected it because the light wouldn't go away. If your light does come on, I would remove the outer housing with the fuel lines attached to it. You may find a lot of gunk in that bottom cavity, I always used a fresh can of brake cleaner and sprayed it into that sensor to blow the debris off of the contacts. Usually the rubber gasket behind there swells, and is very difficult to reinstall. Just use a fair amount of grease or vasoline to make the gasket stick to the housing and reinstall the outer plate.
The sensor is supposedly also removable, but I only ever broke them when I tried to remove them.
One more thing to keep in mind, in the picture above shown as picture 56. That black piece sometimes tries to come out when you remove the outer housing, as there is a spring under it. Just be careful and pay attention when you remove the outer housing.
Last edited by DieselDano; 10-08-2012 at 01:43 PM.