Hi _?_,

>(1) I had the differential speed sensor replaced with a new one. (Polarity >wiring is correct.)

Did the connector fail before? Was it rewired?

>(2) Wiring from the speed sensor to the speedometer was checked for continuity >and was deemed o.k.

You can verify the connector with an ohmmeter by checking continuity with your
ohmmeter between 4-pin connector D, Pins 1 & 2. Brown/White & Green/White wires. With the VSS connector unplugged it should be open.
(above to check for possible short circuit at 2 pole female bullet connector)

>(3) I removed the entire instrument cluster and replaced the malfunctioning >speedometer with one that I know worked previously.

Now you must examine the female harness connectors for being damaged/ spread open. I had this problem with a fuel gauge connector.

8-pin connector A Pin #2 Check pin & Check for continuity to ground.
You can check to the ashtray ground. If it is open, then repair ground 31/6
just forward of drivers door down at floor level.

and 4-pin connector D
D1 Green White S+ to speedo
D2 Brown White S- via unspecified item- looks like a capacitor
D4 Brown Red +12V to instrument cluster onboard fuse 11/41
This fuse common for fuel speedo tach & temp gauges.

Give an e-mail and I'll send you the schematics for the speedo.

Bill