Seat belt warning light?

The timing device is bad--which is very common. Follow the wires from the light and it will go to an oblong relay looking thing, which is the timer. I believe it is located on the side of the pedal support or on the wiper motor mount. They really aren't repairable and they work by heat when the power is supplied in order to open the contacts after about 6-10 seconds. Original timers for a 66 are uncommon and very hard to find. The ones for a 67 can still be found and they will work also. I scored one for a 67 on ebay and it worked great.