On the RadioShark, we soldered the FM Antenna to a copper wire, which was connected to the solderless inline motorla jack from RadioShack. The FM Antenna is the one closest to the edge of the case. It is labled "FM" on the circuit board. I don't use AM Radio, so I didn't even bother with it. When we were done, we routed the wire out of the bottom where the hole was from the screw and put the RadioShark back together.
To open the RadioShark: There's a screw hidden on the bottom of the base. I used a knife to tear up the bottom part to access the screw. It's under one of the f's in the word Griffin. Then you have to unscrew and remove the base. After that, I set it flat on the counter and put the knife in the hole where the screw was and pushed upwards to start seperating the case. I then put a credit card in and started moving around it prying it apart.
I have not tested this in the car yet, but on my desk, the RadioShark used to get kind of staticy, especially if I moved it. Now, with this external antenna, the reception is crystal clear. I think it will be great when I plug it into the car's antenna.