Your answer is: DRILL BABY DRILL Drill a hole!. Check out breedlove mounts. I have one for the trunk and its awesome. Just get a cheap step drill bit from harbor freight or similar. I didn't feel like spending the big bucks on a good one b/c I was only going to use it a couple times in my life. Going through the trunk is easier for me than the roof. I don't have to worry about a headliner. But you should consider drilling a hole. If you think you're going to sell it get a plastic or rubber plug for it and match the paint at auto zone, boom done!