I have the same antenna and had the same problem, but one of the included adapters does work. Just screw it into the antenna first, but not all the way tightened and then screw it into the car. It then tightens all the way into the car without any gaps. It looks and works just fine. Hope that helps, I'm not sure which adapter it was of the four , but one definitely will work.
I had the same problem with my old car. If you are sure that it is the same size for your car and antenna, just take your antenna to home depot and find a matching long screw. Then you just have to cut off the head.Hope someone can help me out here.
I also have hard time to install it (the instruction basically describes nothing).
Here are all the accessories in the package.
It seems the antenna is same size as the hole on the car. However none of the adapters have the same size for both end. (That being said I screw those adapters either into the car or the antenna but failed to screw to the other side).
Most real hardware stores (no, not Home Depot) have threaded rods in all sizes that are either different lengths, or can be cut to length by them. Almost all towns have one or two real hardware stores around somewhere.I had the same problem with my old car. If you are sure that it is the same size for your car and antenna, just take your antenna to home depot and find a matching long screw. Then you just have to cut off the head.
Or you could superglue the smaller end into your antenna.