If you found it on the street, you would be legally required to notify the police, give them as much information about where and how you found it, and turn it over and get a receipt for it. You find out what the legal waiting period is and you go back to the police when that time has passed. If no one has come forward to claim it, you claim it and it's yours.
Because you found this inside property you legally purchased, it's legally yours unless you become aware that the money did not legally belong to who sold you the property. The only way you would know that is when the police come knocking and tell you that the car you bought was used in some illegal activity, and they would be asking you what you found. You would be breaking the law if you lied.
That's very, very, very unlikely to happen. Morally, it's a different issue. You know that what you found is someone else's loss. How you decide to handle it is up to you.
I have found lost property in the past, and how I've handled it is to run an ad on Craiglist saying approximately where and when I found something, and for anyone who lost something around that place and around that time to describe what was lost. In this case, I would get in touch with the owner and ask them if they are missing anything that might have been left in the car, and ask them to describe it. If you bought it from a dealer, get the owner's contact info from the dealer. Don't mention finding anything.