Was going to reply to your original issue until I saw the picture of the buzzer. I know on some of my door buzzers the screw that mounts it to the bus is the grounding screw. If the box isn't mounted your bus won't start. But your buzzer is under the dash, have no clue what it is for.
My bus is very similar to yours, curious to your screen name, what's the E?
Here's mine see the side.