I read somewhere that was for a defroster cable option.
Like others, I put a 12V power point (cigarette lighter receptacle) in mine.
1949 Chevrolet 3/4 Ton - Still Solid. Regular Driver OT Vehicles: 1925 Hudson Brougham Super Six 1950 Chevrolet Styline (Parts) 1952 Canuck Pontiac Sedan Delivery (Well Underway) 1973 F250 4x4 Highboy 1977 F250 4x4 Lowboy