'92-up Crown Vic clusters (and probably MANY others) have a factory 5-terminal "anti-slosh module" inserted in the back which does 2 things:
1) slows down the needle so it doesn't show a drop in fuel level for momentary conditions like an extended turn, and
2) grounds a discrete circuit when the average fuel level is below ~1/4 tank.
This ground circuit is connected to a light bulb that is always energized (B 12V) when the key is on, so when the module grounds it, the light comes on, indicating low fuel.
IF THE IMAGE IS TOO SMALL, click it.

Some other vehicles like T-birds & Cougars have an 8-terminal module which also monitors the oil pressure & coolant temperature circuits. A low reading in any of them triggers the light, which in those vehicles is labelled "Check Gauges".

See the NEXT several captions...

Owner caption July 2006 · Photo 1313 of 5000