I just wanted to expand a bit on the super glue tip. For the best results, be sure to clean both sides of the split grommet with alcohol and glue it before you insert it into the firewall. Gluing it beforehand allows you to get both sides together tightly.
Allow sufficient time for the glue to set up (the longer, the better), before inserting the grommet into the firewall. If you try to glue them while they are already inserted, the rubber is under too much stress, which makes it very difficult to get a clean, secure adhesion.
