How do you get analogue input from PS3 shoulder buttons?

I am playing with the standard assets car. I am using a Mac and Unity 5.6 (I could upgrade if needed, but I thought PS3 control more likely on 5.6 than newer). I have got my PS3 joypad working (the only joypad I have to hand). The left stick is fine for analogue control of the steering. I can get L2 "joystick button 8" and R2 "joystick button 9" to work as the vertical axis, but the input is digital. I would like to be able to modulate the steering and braking, so need analogue input. I have tried all the axis options that appear in the InputManager, but none worked.

Does anyone know how to do this please?