Nice project, and I think you’ve done a great job.
I’m afraid to say, it won’t be appropriate to suggest a position for the placement of ciruit untill I know the details. I’ve no idea about the sensors you are using for checking the brake, no idea how you are checking the AC etc.
Anyway what I understood is that you have a main board, which receives SMS, and read the data from other sensing boards which are linked to the main board. There must be another board which can start the engine by receiving control signals from the main board.
If it is so, I think the sensing boards should be placed close to AC, Breaks etc, engine starting board to be placed near the start key wires. It would be better to place the main board somewhere near the dashboard.