it'll be the microswitch on the boot button

tricky little fucker to get one as you have to dismantle the little bits that get in the way in order to remove the boot trim it's located in

when you pull the boot trim panel off there should be 6 10mm or 11mm nuts holding the panel in place, but it's complicated because there are wires and stuff for the rest of the boot running along there and the number plate lights can be a right pain in the arse to get apart as they may have rusted into place

the boot switch is a tiny little wire you unclip so take it all apart and see if it just needs greasing or a new switch before you order a new one