If you are working alone and can't do the string trick, we used to "V" the grommets we were putting in electronic chassis. You press one edge of the grommet in towards the center, so it looks like a circle with a V in it (V). This effectivly decreases the diameter of the grommet so it will go in the hole. squeeze the legs of the V together when putting it in the firewall. Once in, a pencil is used to push the V away from the center to form an O again. Good luck.
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E