Which Diamond? Some of the antennas they sell are no-ground needed (I have one, and it's pretty good from about 120 mhz up to about 170 mhz and 430-500 Mhz or so).
Probably the best way to ground the bracket would be to put a copper braid strap from the bottom of the mount that should bite into the trunk (or whatever you're putting it on) to a known ground point - and that will take a little hunting with a VOM. I'm not familiar enough with a Saturn to say where that might be found - but wherever you do this, the shorter the braid, the better. Since this is a receive only app, I can't imagine where more than 1 or 2 would do the job nicely. 73s Mike