StrOkEr , some thoughts on what your doing...
1. Is there enough fuel in the tank to register on a gauge? The stock gauges would bottom out when the power is off & come up to Empty with power on even with an empty tank but I don't know what to expect from the aftermarket gauge.
2. Have you ever removed the sending unit to inspect? There are three basic components to a sending unit, the float that must be good & free to move, the variable resistor that the float varies to change the power level received by the gauge and the pickup tube with filter (sock). If you have never removed your sending unit for inspection this would be a good time to do so if for no other reason to make sure the sock is intact.
The resistance test Mike has explained is right on. If you have no experience using an ohmmeter I suggest you find something working that you can experiment on to familiarize yourself with to know what to expect. A speaker typically has 4 to 8 ohms resistance.
