I think you're hooked to the wrong wire. My '93 truck schematic has the light blue/black wire going to the speedo.The dark blue wire is the Rear Wheel Antilock Brake wire. If you pull the instrument cluster and look at the plug, there are letter designations for all the terminals on the plug. Letter designation "S" should be the light blue/black wire. This wire is the speedo wire. You can either snip it here and run your green wire directly to the plug or snip it at the buffer box being sure to eliminate the buffer box from the circuit.