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The tachometer on your 92 Dakota is driven by what's called a tachometer module, which is located on the back side of your instrument cluster. You probably saw it... it's the finned unit.
What you need to check at this point is to see if your engine controller (PCM) feed is good between the red instrument cluster connector and Pin 43 of the PCM 60-way connector. The circuit you're looking for is colored grey/ light blue.
The PCM 60-way connector is numbered under the wire shroud directly on the connector mold, which seems to be the best way to determine pin locations. If you find that continuity between PCM and cluster is OK, chances are pretty good that you'll be needing a tach module unless continuity on the printed circuit board has been lost.
These boards are actually more like plastic sheets with copper inlays, which don't always maintain their continuity between the pin rivet (that connects to the harness) and the copper conductor. Follow your circuit through the board with an ohmmeter all the way to the 7-conductor connection point of the tach module to be sure it's OK. If continuity is lost, you can probably get creative with a pencil tip soldering iron to restore contact.
One last thing to check would be the power source supplied to the tach module, which would be the only red wire on the red connector at the cluster. It's not clear in the diagram whether this wire is hot with the key off, but it should certainly be powered when turned on. Check fuses if no power is found or check back with me and I'll track down the point or origination.
Edited by Dodgerench on 1/24/2011 at 8:05 PM EST