One test you can do:
At wire connector outside the fuel tank access plate, check for voltage in the sender wire.

Gauge sends 10.2V to the level sender. I don't really like to call it a sender because it doesn't really send anything; it receives voltage and provides a resistance to which the gauge responds. The 10.2V is provided to gauge by the voltage stabilizer on rear of cluster. It also powers the temp gauge, if no volts from that then both gauges will read at bottom of scale.

There are three wires at tank access plate: hot 12V to in tank pump, 10.2 V to level sender, and ground. With key in position II and engine not running the only one that should have voltage is the sender wire because the computer cuts power to pump after a couple seconds if engine is not running.

If you find power there then you can ground that wire (briefly) and watch the fuel gauge needle rise quickly.

Also I've heard that the gauges are grounded via their mount screws so make sure they are tight.
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Sven: '89 245 NA, 951 ECU, open-front airbox, E-fan, 205/65-15's, IPD sways, E-Codes, amber front corner reflectors, quad horns, tach, small clock. Wifemobile '89 245 NA stock. 90 244 NA spare, runs.