Well it really depends on what year your contour is I have a 1999 contour SE and mine is in the wing that has a few screws that have to come out and you can pull the light assembly out and change the bulb. But if yours is older and its behind the rear window take the plastic cover off usually it has one or two screws holding it in place unscrew it pull it back or up being easy with it and you can now change the bulb and now when your done put the cover back in place put the screws in place and snug the screws tight Warning do not over tighten because the plastic will break or crack very easily and now your done . This should take you less than 5 minutes to do and its that easy.
Begin by removing the wiring harness from your third brake light. Remove the third brake light retaining screws. Reverse the process to install your new third brake light.
the third brake light is direct from brake light switch no fuse
I am not sure on a ford but i had about the same problem on a GM car i had and it was the turn signal switch i replaced it and all the brake lights worked fine you might try that
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The third brake light is named for then Secretary of Transportation Elizabeth Dole and are called "dole lights".
Chevelles never had a third brake light since the Chevelle name was dropped after 1977. However, third brake lights were first installed (and required) in 1986.
with a hammer
A third brake light is used to provide a deceleration warning to vehicles that can't see the left and right lights. You can learn more about these brake lights at the Wikipedia.
New bulb needed or broken wire
Crawl in the backseat, remove the screws on each side of the housing, pull off the light replace the bulb and re-install! Good Luck!