what causes the low coolant light to come on when radiator is full

Might be something wrong with the sensor.

My sensor is fine, the fuse boxes in the block are rotting from the bottom. The only thing I can find wrong with it, and I can't find a junkyard with a Lumina or Monte with the fuse boxes in good shape, or in them at all! Seems to be a regular issue.

find your plastic jug usualy on pasanger side ,top will say engine collent only look on side you will see 1,cold with a line,2go up will see full hot.check this cold and fill with collent to full cold.hope this helps.PSdont do this with engine hot,you can just dont remove raidor cap then fill to full hot.after cools down see where it is showing.