I'm not sure exactly what you're trying to do, but if you remove the light bulb from the instrument cluster, you don't have to worry about the light. It's a simple DIY.
Is the female part of the seatbelt still on the car? If so, you can buy a plastic dummy to clip into it, this will act as a 'real' seatbelt and cancel the light/alarm. Otherwise removal of the bulb will be the second option, third option is to cut/bridge the circuit at the seat side plug depending on how the circuit works normally open or normally closed.
are you completely removing the original seat belts from the car?
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I still wave at Boxsters, but they no longer wave back :-(
2002 Boxster S "Violet" (sold but not forgotten)
2009 Carrera 4S "Kelsey" (current ride)
2015 FIAT 500e "Nikki" my commuter car