The FSM shows that the tach and the fuel gauge share the same grounding point namely G201:
G201 is located behind the radio. It will look like a single black wire with an eyelet attached to a black vertical metal beam that supports the center of the dash. In reality, that one wire leads back to a splice called S212 which has 16 black wires going into it, one of them leads to the tach and fuel gauge. If you follow the single wire from G201 into the harness towards the driver's footwell, you'll find S212 buried about 6 inches in. It will look like an octopus of black wires covered in electrical tape. Sometimes the acidic solder paste that was used begins to break down the splice connection and gives problems. Since nothing else is acting up, that's where I'd start.