Today, I took another pass at fishing the yellow wire through the firewall. I had to buy some wire hangers and that helped tremendously. Using the coat-hanger wire, I was able to feel my way through the inner boot. It certainly wasn't any easy route. I figured out that the angle of the wire going in is only about 15 degrees above horizontal. It felt about like I was blindly threading a needle.
Once I got the wire through, I still couldn't see it under the dash. I ended up having to remove the 3 nuts from the X16A junction box and pulling the box away from the firewall. It didn't move much, maybe an inch, but it helped me see the coat-hanger wire sticking through. I was able to stick my hand behind the X16A and guide the coat-hanger out the left side. I taped the yellow wire onto the coat-hanger and gently fished it back through. It works! Hallelujah, praise the Lord!