Well, after further review...
I was in the process of running a shunt wire in parallel with the wire in question when I noticed that it runs through the big connector on driver side of the firewall (C100). I decided to disconnect, clean then reconnect several times to "wipe" the contacts, and with great joy I found that the voltage drop mostly disappeared and the flicker seems to have gone away.
I'll see if this lasts, but it looks good for now.