Bindings=(Name="L",Command="GBA_ToggleVehicleLocking")
....
Bindings=(Name="GBA_ToggleVehicleLocking",Command="VehicleLockPressed | OnRelease VehicleLockReleased",Control=False,Shift=False,Alt=False,bIgnoreCtrl=False,bIgnoreShift=False,bIgnoreAlt=False)
Make sure these lines are intact in your UDKInput.ini or just do what Sn4ke said.