Welcome to the forum Dzehr!
Are you referring to a pressure sensor in the seat cushion of the second row seats?
If so, I don't believe there are any but I'm not entirely certain of this.
However, are you perhaps being bothered by the "Rear Seat Reminder Function"?
From the "2023 Highlander Hybrid Owner's Manual", page 129:

 

Not that reminder. There is a way to turn that off thru the settings. I am speaking of the seat belt alarm in the middle and 3rd row. If I have adult passengers who don't want to wear seat belts, they shouldn't have to and I don't need to be annoyingly reminded of it.

 

There are no airbags controlled by the presence or absence of second or third row passengers, so that's not an issue.

I don't know if simply disconnecting the wiring will solve the problem, though. It depends how they're wired, and if inserting the buckle opens or closes a circuit. I would guess that it closes a circuit, which means disconnecting things won't help.

Not condoning this practice, but you'll find buckles on Amazon for just this purpose.

 

There is no rear seat thorax airbag, like the front seats, or the 2021+ Sienna 2nd row,

 

Buy a seat belt silencer on Amazon. A two pack is about $12. Stick one in the female end (anchor) on any seat that's occupied by someone to ignorant to wear the real belt.