Low Fuel level light problem.

[Sandor]

Car is a 2001 1.0 16v. Came out of the factory with the clock without the rev. The low fuel level light was working good. Then I swapped the clock for one with the rev, from a 1.3. Now the low fuel level light does not comes on. If i remember well, there was a beep sound too when the fuel level reached a certain level, and it sounded when the light came on. Now there is no light or sound. Any help? Also my fuel gauge is not perfect, its acting up strangely. Goes up and down intermittently. Any advices will be much apreciated.