1992 Miata backup lights out

Hello,

Both of my backup lights are out on my '92 Miata with about 45,000 miles 
on it.(stored winters in Buffalo,NY area never in snow and rarely in 
rain....no rust....clean underneath)

Does anyone know of a most likely area to begin investigating the cause 
of the problem?

Anyone it happen to them in an early Miata?

Thanks for your help,

Mike