The "grommet" in question is actually a compression type fitting called a gland seal. It will have a flange and flat seal on the insde with a male thread then another flat seal on the outside with a nut, these can have a smear of sealant (SIKA marine sealant, not silicone based). This holds the sleeve in place. Then you pass your unfinished wire through and slide the seal (which will be tapered) down into the (also tapered) end of the sleeve and then fit the outer nut. As it's done up it pushes the seal into the taper (which can have a smear of the same sealant) and seals against the sleeve and cable. Work very well, especially if you just start it and then with a syringe and needle squirt some sealant in between the cable and seal.
Here's a pic https://www.aulro.com/afvb/
and an exploded view, not a perfect example but you should get the idea.https://www.aulro.com/afvb/images/im...011/08/395.jpg
Nut (in this case retains cable), body, seal outer body (male thread into main body through seal) and outer nut. The fingery parts on the outer body has the rubber seal inside which is compressed as the taper inside the outer nut squeezes the fingers in against the cable.
Hope this has helped.

