FYI ...
If the Deluxe Belts was a "factory installed option" on a "non-tach / standard instrument cluster" car, the seatbelt light is in the instrument cluster (RH side of FUEL/TEMP Gauge Pod), not on the lower dash like it would be on a factory tach cluster car.
The harness for the dash cluster belts light is different in that it has a twist-in plastic bulb socket (see pic) and is likely more common.
Just thought I'd throw this out there too.
Doug