I've got everything unplugged / cut (previous owner botched a radio install) but I'm not sure how to go about getting the wires through the firewall grommet, and the shop manual was no help. because of where it's located i haven't really tried brute force, but is there a trick, or is it easier just to cut it out and buy a new one?

 

I don't work on that many 69's but generally speaking the grommet is attached to the wiring. So the grommet will need to come out with the wiring. What ever side of the grommet is bigger, use a screwdriver from the other side and push the grommet through with the wiring.

 

Are you talking about one of the two main harnessess in the firewall? If I am thinking about the right ones, that is a plug, not a grommet per se. The plug pulls out from the engine side. The passenger compartment side is held in by two metal tabs that you will see and easily push in to remove the other half of the plug.

http://mustangefiswap.blogspot.com/

 

The plug lines up with the fuse box, the part i'm at is for the wipers and i'm not sure what else, and it's several three different plugs just inside the engine compartment from the firewall, and I couldn't budge the grommet by hand, and looking i couldn't even tell how the wires / plugs would get through it even if i could get it loose

 

It would seem I am guilty of assumption.

Your best bet is to get a wiring diagram for that car. That way, not only can you find out what each wire is, but you can see how they installed it. The wiring diagram for my 65 shows the types of plugs they used.