The seat belt warning system was part of the courtesy group option, I don't think it was a separate option. The relay is thermally activated which turns off after a few seconds, but the bi-metallic strip still has 12V going thru it while the car is running. The major problem after 40-45 years is the contacts gets pitted and the lamp doesn't light up, and the contacts may even fuse together, lamp won't go out. There is a difference between the 65-66 and the 67 on types only in mounting. Expect to pay $25 plus. Service part number for reference is - D1AZ-10B924-A and the housing is marked C7AB-10B924-A or B, or D2AB-10B924-AA. The 65-66 service number is C6AZ-10B924-A.
Jim