How do I repair the guages in a 1986 Chevy Suburban?

Hello, the name is John. I am a driver for a non profit org in fort lauderdale fl. We currently have a 1986 chevy suburban we haul teens around in.
We are having a problem with all of the gauges, none of them are reading anything, except the fuel which jumps and is never truley accurate, we've already stopped once w/o fuel, and there is also an after market oil temp guage that was installed and that works well. I was wondering if anyone could put me in the right direction as where to start repairing , or what wires to start looking at first. Thanks
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