Crawl under the dash and check the ground wires. If it's anything like mine, the grounds for the guages are daisy-chained together. If the ground gets loose, you'll lose the whole thing. Also, check the power connection at the ignition switch. Mine has a separate wire that powers the instruments. If that doesn't work, it's time to break out the multimeter and start tracing wires.
DKJ
1992 Supra Mariah - Red
PCM 351HO 285hp - PCM 1.23:1 Transmission
OJ XMP 4 Blade CNC 13x15.5RH
"People do weird things to boats" -Unknown