Backing lights don't work

I need assistance "if you would" in fixing my 84 Vette backing lights.
A fews day back my son brought to my attention that my backing lights don't lit as I am backing-up.
I replaced the light bulbs, even though they were not burnt (a new set of light bulbs won't hurt my Vette) but they still don't light up.
Any suggestions/ideas will be greatly welcomed.