I don't think the seat belts have anything to do with your problem. There is a switch inside the warning module which could be stuck closed. Or there is a problem in the light switch that is fooling the warning system into thinking that the lights are on. (The warning should activate with the parking or headlights only, not the standing lights or with the lights off.)

The "lights on" warning is grounded through the door switch which powers the courtesy light. It's wired separately from the seat belt warning.

Does the seat belt warning light in the instrument cluster come on when the door is open? If not, it would further confirm that the seat belts are not part of the scenario.