Sounds to me like...The dreaded "white" wire overload syndrome...

The main large white 12V feed that goes to the fusebox from the engine bay gets overloaded.. This can melt and corrode the connectors that join the enginebay loom and the underdash loom. It can also result in melted fusebox connectors.

Have look under the drivers side of the dash, and disconnect and inspect the condition of the 2 plugs that feed them loom through to the enginebay.

If any of the connectors are corroded or melted, you'll need to replace the connectors which isn't a trivial job.

The reason the light comes on is because of changes in resitance to the 12V feeding the dash guages. The sensor units supply varying levels of resitance to earth.

I suspect corrosion in the loom connectors.

Sometimes the round multiconnector that connects to the back of the dash can also get corroded, wiggle that connection on and off to freshen up the contact.

Chris