Copy the .lua file into the scripts folder in your Mafia III directory (Default for steam is C:\Program Files\Steam\steamapps\common\Mafia III\scripts).
Remove the .lua file from your scripts folder.
Default keys are:
"Question Mark (?)": Turns engine off. To turn it back on tap "W"
"Comma (,)": Turns headlights on.
"Period (.)": Turns headlights off.
Simply edit the .lua with a text editor and change the keys to your fitting.
1.0 - Initial release
1.1 - Changed default keybinds. Check above or in ReadMe