Thanks for the suggestions.
The string method wouldn't work since I'm working solo right now. John, poking one side of the grommet wasn't viable on this one as the grommet was recessed behind/under a large flashing which conforms to the firewall, plus the grommet was fairly thick.
I thought about this some today while working, entertained the idea of cutting a slit in the edge of the grommet and screwing it in. Figured with the flashing, and proper positioning it should be water tight. Opted to try something else, but similar.
I pulled back the flashing and put a dollop of assembly lube in the grommet channel. Then I pushed it into the hole slightly askew and started turning. The lube made all the difference. The grommet screwed in and seated flush, nice and neat.
Pbalis - you'll probably have to disconnect one end of the cable(s) and slip the new grommet on, slide it to the firewall and install from the engine bay. In that situation, JCE's suggestion of poking one side in should work.
Thanks again for the suggestion, and I hope my method helps others in the same boat.
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