The Possess Bar usually only uses 10 buttons to the limit of 10 buttons on the pet bar, that leaves button 11 and 12 empty. Button 12 (id 132) is now being used for a Leave Vehicle button to be able to leave vehicles properly.
Without a full 12-Button Main Bar, you probably want to Macro /script VehicleExit();
I may add another form of button for this at some point, but not right now.
Hendrik Leppkes