Need Help With a Stereo Antenna

I decided to remove the fender antenna (for the smooth look), and my original thought was to install a sharks fin antenna on the roof. After I thought about it for awhile I decided not to do that. So here's my problem: I need an antenna that I can install in the dash in an area about 6X4X4 where the speaker grill used to be. I'm using the box area for my speedo GPS and there is room for a small radio antenna. It would see the sky so reception wouldn't be an issue. I know a Sirius satellite size antenna will fit along side the GPS unit but I don't know if it will receive anything other than satellite signals. An other possibility is to get a shark roof mount antenna and modify it to fit in the box area. Any and all thoughts/help would be appreciated.