i'm not sure there is a gromet. isn't it more of a gluey substance that goes around the cable as it goes through the firewall? i'm not sure and i haven't looked at either of my trucks in a while.
Mine had what looked like a padded tube split down the middle. The kind you find on your AC units' lines next to the house. Stuck out about an inch on either side of the firewall.
thats what i was thinking they had on them. maybe you could get something similar with a piece of pipe winterizing foam and just cut it thinner to fit. those foam tubes have a peel and stick tape on one side of them that can be used to hold it back together with.