Hyundai Repair Questions? Ask a Mechanic for Answers ASAP
There is no way to get into the trunk without tearing up the vehicle - either cutting a hole in the rear seat metal support that goes separates the trunk from the interior or from using a large prybar on the trunk lid.
As this is a rental, I dont believe you want to go that route. You need to call Hertz and have them send someone out with a duplicate key.