The tailgate open switch is part of the latch assembly. There is no reset but it's possible that the latch is not closing completely and needs to be adjusted or the switch got damaged.
Try this too, there are 2 rubber bumpers on the hatch to keep it from rattling. If they are mis-adjusted it might put enough pressure on the latch to activate the hatch open switch. Try turning the bumpers in a half turn each and see if the problem clears. If it doesn't fix it turn them back out. If it does fix it but you get rattles the turn them out a little at a time unit the rattle goes away.