Are you working with this type of connector?
If so, the procedure I use to install them is:
(1) Working from the engine compartment side, put the grommet into the firewall hole with the tapered side facing inward. Make sure the grommet is fully seated in the hole.
(2) Working from inside the cab, push the cab half of the connector into the grommet. The outer ring on the plastic connector shell should snap into the grommet's inner groove, securely holding the connector into place. In fact, once these are snapped into place, getting them back out can be very difficult (especially with an older grommet that isn't as soft and pliable as a new one). Warming them up with a hair drier (or heat gun on low setting) can make the grommets a little easier to work with if necessary.