The usual problem is the thermal relay used to time the seat belt light is bad. Here's how the system works:
When you turn on the ignition (ON or ACC) power goes to the thermal relay. Its an NC relay hence the seat belt light comes on. After the thermal relay heats up sufficiently, the internal contacts move from the NC position to the open position. This action breaks the connection to the light, so it goes out.
Why doesn't it work:
1. It could be a bad light in your instrument cluster. Replace with a known good lamp and check the connections in the lamp socket. If all is well, look for the thermal relay. I think in a '66 its under the drivers seat.
2. It could be the thermal relay has been removed by a previous owner. This would make the lamp go out and stay that way. The owner may have done this to prevent the light from staying on all the time. Look to see if the relay exists or if its been removed.
3. If you install a new lamp and the lamp comes on but never goes out, it could be a faulty thermal relay. When they go bad, the light stays on all the time which is why some owners just pull the relay and forget about using the seat belt warning. These thermal relays are not made anymore and finding one is like finding elves in the forest...not easy! These can be successfully substituted however. To do it requires electronics technician skills however.
Hope this helps...
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