I've got a newer rig, and had to go look at how the 89 did things. It's a pretty simple circuit, and the White/Black wire that powers the tach also powers a bunch of other things including all the other gauges, so if they're working that's probably not the problem. The Orange/Black wire powers the dash lights, and it all grounds through the black wire. Assuming the two power wires are energized when they're supposed to be, I'd try a jumper to ground first on the case of the tach and then tapping the black wire at the connector. That's about all that could be wrong with the dash lights. The green wire supplies the tach signal from the ignition module... not sure where that is on your rig.
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