So I tried adding parked cars support to an old-ish mod map, but they just don't appear. I have placed parking spots to the map through the sites editor, but cars just don't spawn in. Also after I save and exit the map and come back again, all the parking spots have disappeared like they never existed. Am I right that it should create a city.sites.json file to the map folder? I can't seem to find that anywhere, so it probably hasn't created it. I am pretty new to this, so it might just be that I've screwed something up.
You have to save the file, and than move it into the main folder of your map, name it city.sites.json. Worked fine for me.