I did some checking. Rang Redarc and spoke to the technical section. He said that generally brake controllers or operational parts should be mounted in a position that can be manually operated from the drivers seat, but check for state rules and regulations. Also check the link to an artical from a magazine, rules and regulations, 3rd paragraph. I have no idea on how things were done 15 years ago, just from what I was led to believe at the time I installed my controllers. You can get newer Bluetooth controllers which might be different in mounting them but think they still have a controller in the car.
https://rvdaily.com.au/buyers-guide-to-electronic-brake-controllers/