1) find 3 same voltage transformers
2) build the baseplate with connector
3) buy the components, prototype board, and solder it
4) make PID or increment/decrement regulator sketch
5) try it with adjustable transformer and lightbulb
6) install it into the alternator junction box, try it at idle up to full power, then do some measurements with startup surges from overvoltage-proof equipment and then use it.
optional) make many more functions into it.