Ok, that's interesting that the tach works, I'm not sure what to make of that.
Just follow your Chilton's for getting the dash apart. Once the dash is off there should be four screws that hold the cluster in, it comes out all as one unit. I believe the lights on the side (for e-brake, seatbelt, 4x4, etc) will be attached and it will all come out together. At least that's how my 89 is, but they changed a lot starting in 91. Then there should be two harnesses, one for the warning lights on the left and the other for all of your gauges and backlighting.
I would take a look at it visually, to begin with. A multimeter would be handy to have, though. For instance, you can measure the voltage between the orange/black wire (which according to the diagram is the gauge illumination wire) and ground (I use the outside edge of the cigarette lighter) to see if there is any voltage in the harness when you flip your lights on. Then you know if it's a problem with the cluster, or in the wiring.