I didn't think any of the big carriers would be arriving that late at night. If there are big planes still coming in, the car rental guys will still be there. But regardless, you should make advance arrangements with whoever you book through and if so, they will be there to meet you when you arrive.

[edit] Now that I think about it, unless you are staying on the French side, you might be better off to just get a cab to your hotel. SXM car rental guys rent their cars based on the calendar day, not by the hour. So, if the cost to get to your hotel that night is less than the cost for 24 hours for the car, it might be better to just take a cab to the hotel and then have the car rental guy deliver your car to the hotel the next day. But take into account that the cost for a cab goes way up--like double--after approximately 9 PM.