got an ohm meter or test lamp?
#1pull wire off thumb switch and insulate the end.. do not let it touch ground.
be in neutral to test this just in case you mess up on #1.
#2, get ohm meter.. one lead to chassis one leat to tab on switch.. press switch.. should go to 0ish ohms.
or #3, test lamp from bat hot to tab on switch.. pressing it makes the lamp glow.