I realise my viewpoint may not be popular, but rear parking sensors are not so important on a car such as the 500.
I accept that spatial awareness isn't something that everyone finds easy, but imo, rear parking sensors are only really useful on cars with a proper boot, or an estate vehicle. For a tiny hatchback such as the 500 with quite literally nothing sticking out much past the rear screen, a quick glance over your shoulder reversing into a space should be more than enough to see that you're not going to suddenly bang into a wall. Please excuse my crudeness, but sat in the drivers seat of a 500, you can turn round and spit on the back window, they're hardly the length of Mercedes Sprinter! My wife owned two 500's neither of which had reversing sensors and she didn't find reversing without them an issue.