I have attached a check out to see if it is your sensor (though you just replaced it) or the speedometer it self.
I would really look at the wiring really good.......(I know you said you did) at the plug for any loose pins right there at the VSS itself. Or is a wire loose in a pin that fits into the plug. If it only works when it is raining it makes you think the rain is providing better continuity.
Look at the wire really close for any damage, kinks, bare, chaffing etc the entire length.
You might also consider removing all of the parts that the VSS connect to. Inspect and clean. You may need to replace the O rings though.
Remove speedometer assembly.
Check for continuity in the wire between the distance sensor connector and the speedometer (distance sensor input pin).
With the ignition switch in the ON position, check for battery voltage across the B+ pin and the Ground pin.
Perform distance sensor test.
If all of above tests prove good, replace speedometer.
Thursday, December 18th, 2008 AT 6:45 PM