A kit for assembling a backrest on a saddle-type vehicle seat selectively connected to the frame and including at least one seat passage extending therethrough. A mounting plate is adapted to be disposed between the seat and the frame. An adapter, adapted to be connected to the mounting plate or integrally formed therewith, includes at least one adapter passage having an open end facing away from the mounting plate. An inverted U-shaped seat mount includes two posts and a connection member connected between their respective first end portions. A second end portion is adapted to be selectively received in the at least one adapter passage through the open end thereof. A back support is adapted to be pivotally connected to the seat mount. At least one fastener for attaching the mounting plate to the seat. Methods for assembling the backrest onto the seat are disclosed.