how do i remove the passenger back rest from a 07 honda gl1800?

Answer 1

The passenger back rest is bolted to the trunk. Open the trunk and unbolt it.
Do you mean the little flap at the rear of the seat? It's not removable unless you want to cut it off.
Or do you mean the riders back rest? That bike doesn't come stock with one. It's after market. It's probably bolted on at the seat's grab rails.

Answer 2

If your car has adjustable backrests there is a release button near the top of the seat back where the headrest is inserted.