Brake lights/backup lights not working ?

87 Corvette....118K miles...
I'm having a problem with brake lights and backup lights not working. I've checked the obvious --- bulbs, fuses, etc., and now at a loss as
to what could cause this problem.
Could this be a switch problem? Although I'm not aware of any switch that would control these functions, perhaps someone has experienced similair problems.
Any help would be appreciated.
Thank you,

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