Forest,
When I installed satellite radio in my Wife's SUV, I didn't want to mount the antenna on the roof, so I discussed alternatives with a tech at Monster Cable. He said there can be no metallic objects between the antenna and the sky. He also said, the higher the antenna is mounted, the better it will work, and that the more horizontal the mounting surface, the better. I ended up mounting it inside the SUV's hollow rear spoiler and it worked fine although not as well as the installations that mount the antenna on the roof. As an alternative to mounting it under the dash, if you have the factory plastic front hood vents, you could make a horizontal bracket that mounted the antenna beneath one of the vents.