If you can get to the inside of the firewall where it passes through, you can try the old glass gasket trick.
Put a light film of lubricant on the groove of the rubber grommet, as well as the end you are trying to run through the hole. Wrap one loop of string around the groove. Pass the two ends of the string through the hole. Have a buddy start one side of the grommet's groove into the hole where the string comes out of the groove. Now, while your buddy is applying pressure to the grommet, gently pull the string to guide the lip of the grommet through the hole.