Are both your gas and temp gauges not working? If that's true, you need to check the voltage limiter.
If your temp gauge is working, the limiter is good.
Sounds like the voltage limiter is blown or something isn't hooked up right. You should have +5v at the fuel and temp gauge... that's if the rally dash and stock dash are the same electrically speaking.
You can test it this way:
Remove the limiter from the instrument cluster.
Connect it to a 12v source. You can use the battery. Make sure the 12v and ground are on the correct terminals.
Connect a voltmeter to the output and ground terminals. If you have an analog voltmeter. the pulsing output voltage is easy to see. With a digital meter, the voltage reading will jump around a lot.
If the meter reads either 12v or zero, the limiter is bad.