The problem isn't the missing pin on the molex connector, the problem is that there's no corresponding position coming out of the sled docking connector for that spot. So it's more than just "not populated", it's literally not available at all for connecting the inside of the player to the outside world. So you'll have to hijack a different wire from somewhere else on the docking connector for that purpose. The cell phone mute wire is a great candidate for that, I think, it just requires more complicated surgery.

Found another copy of the sled wiring diagram in the BBS history to help illustrate. It shows the cell phone mute wire as a light green wire that is connected to serial plug pin 1, and the headlight illumination sensing wire as a gray wire (actually white in reality) connected to serial plug pin 8.