What should I do with a sum of money I found in the car under the seat?

If you find a sum of money in your car that does not belong to you: if it is a small amount, you may take it and benefit from it. But if it is an amount that people of average means would actively seek out, then it is a found item (luqata). You should publicize it for a year, then you may donate it or benefit from it while guaranteeing it to its owner should he appear. However, if you find it in someone else's car, the original principle is to hand it over to its owner, unless you know that it does not belong to him, in which case its ruling is that of a found item (luqata).