The connector comes off (it just screws on and off with threads) the end of the coax going into the cab. You go from roof under plastic bracket, through grommet, down into the cab. The ring probably would not fit into the existing hole. Be gentle with the coax with feeding it through. I may have broken the continuity on mine because I was not very cautious and it didn't work after installed (continuity was not present from one end to the other).
Let me clraify...the end comes off easily on this particular cable (FireStik MU8R18):
WalcotCB.COM EZ Install Coax