Hyundai calls it a sunroof, yes. But that's a glass panel, which is what Lincoln Mercury trademarked as a MOONROOF and properly that is still called a moonroof. A sunroof would be less of a problem, as the metal is still capacitively coupled to the rest of the roof. With a glass MOONroof, it simply isn't going to be counterpoise.
What you've got, if you try different locations on the metal aft of the moonroof, is that the A, B, and C pillars will be your effective ground radials connected down to the body of the car. Sometimes a difference of a few inches makes a big SWR difference. Sometimes, the antenna setup or the way you're using the SWR meter is an issue. If it is all unacceptable, look for a "no ground plane" antenna for the car.