I've personally never had them on the two 500's I previously owned. If you use the search forum function, you will find threads relating to parking sensors that have come up before.
It's a personal opinion, so please don't feel offended, but I can't personally see the point of parking sensors on a car with a flat ass so to speak. You have the back window and there ain't much sticking out behind it. In our 4 years of 500 ownership, neither me or the wife had any reason to want reversing sensors fitted to either car, it just seemed utterly pointless. But, I accept some folk do have issues with parking, reversing into spaces where you're wondering whether you're getting too close to another cars bumper as you swing in, so fair enough.