When trying to feed wires thru an existing firewall grommet or fitting with a tight bundle of wires already filling it, gently work a thin philips-head screwdriver into the bundle, careful not to nick any wires.

Then, feed a plastic drinking straw (or those plastic coffee stirrer straws) along the screwdriver shaft.

Or, just slip a straw over the screwdriver shaft, shove it in, and pull out the screwdriver.

Presto, you now have a slick wireway to feed your new wire thru the straw. Pull the straw out once you're thru.

This works especially well with really thin gauge wire that is not very rigid to start with, and that tends to fold over and bunch up when pushed from one end.