Ok, so I just got things installed and
man was the firewall a doozy. Never run stuff through there before. Sorry for the long post but someone may find this interesting.
I originally tried the plug to the left of the main harness grommet but quickly ran into issues. My radio's power wires have a fuse on the positive/negative ends that are pretty thicc. It's already 14 gauge wire and now we're throwing bulky plastic into the mix. I didn't see an easy way to route that downward through the smaller hole, and putting a hole in the interior rubber seemed to complicate things.
So I instead looked at the main grommet and noticed there's a "nipple"/dome that has a sort of hollow rubber conduit on the other side. I sliced off the head of the dome, found some aluminum wire laying around my house, attached one end to the ring terminal of the power cable and fished it through from the interior to the engine bay. You'll notice from the attached photo that this resulted in a less-than-attractive inside-out conduit thing I just described that I'll have to correct and seal tomorrow. The first cable was a pain with the plastic thing but eventually I got it, the second cable was even more of a pain with the first cable taking up some space, but I got it through.
Finally I had to fish the antenna wire through. What I was working with is basically this (C101 with mini UHF connectors):
DIAMOND C211 Antenna Accessories Mobile, C211
At this point I realized my mistake of deciding to route the
thinnest cable last. In the future, definitely start with this one first. I ended up tying a loose knot in the cable and wrapping my aluminum wire around that, then fishing it through. Once it was through I pulled hard on the female end of the extension wire, praying I wasn't going to damage the cable. Finally got the female side through and we were in the home stretch!
From here I ran the cable up the windscreen gap to the roof weather stripping gap, and finally to my Diamond K550 (
DIAMOND K550 Antenna Accessories Mounts Luggage, K550) antenna mount on the roof (neither of these gaps is a tight fit fwiw, I Gorilla used tape for now).