Ok, so this small project got put on hold for a while but it is now finished. I wanted to update this thread with some photos because when it comes to this stuff, I'm much more of a visual person and this might help someone else out.
REMINDER: This install is on a 1990 and I'm not certain if it is the same on other years.
I located the C100 Terminal in the footwell. There was no need for me to feed any wires through the firewall. I took the best photos I could as you're pretty much twisted up like a pretzel at this point. It is directly to the left of the clutch pedal.
I located the white tach wire that leads to the instrument cluster. This was located in the E1 slot (I believe it was E1... it is the only white wire I saw) on my '90. The slots are actually labeled, but they are labeled super tiny but I could see them with a flash light. I spliced into the white tach wire and connected the green tach wire from my auto meter.
I then grounded the shift light underneath the instrument cluster right next to the radio. I don't use the cigarette lighter so I ran the shift light power to it. The autometer control module just sits inside the center console. It came with double sided tape but it really cannot move. Everything works perfectly and I hope this helps someone else.
Cheers!
Jay