The antenna needs a clear view of the sky.  They are usually mounted on the
roof.  An installer I met through this newsgroup said he's had success
mounting the antenna on the rear package shelf in sedans and coupes.  I
tried that when I installed my Sirius tuner and it worked very well.
Eventually I moved it to the roof per the installation instructions.  There
was a slight improvement there.  I assume the antenna you have is a magnetic
mount.  Just try running the antenna wire through a window and sticking the
"mouse" on the roof.  If your reception doesn't improve there's probably
some other problem.
 Good luck!