Why does my speedometer dim in certain conditions. I want it to stay bright all the time. What do I do?

 

You don't say what the certain conditions are - but in every car I've ever owned (including the current Ford 500) the dash lights have dimmed whenever the headlights come on.

EDIT: Look for a dial controller that adjusts the dash light intensity <within the dimmed range>.
The dial/switch is probably close to the steering wheel column; on my Ford 500 it's on the dash, 6 inches left of the column.