389 Marker light??

I just looked at the schematic and yes, the marker lights are tied together somewhere in that chassis harness, but I do not know where. Be fore you go too far, grab that wire at the plug by the bumper and give it a good tug on both sides. That is the most common place I've found problems. It's common for the wires to break inside the insulation right at the plug, if that's the case, you'll be able to pull it apart there. If that's all good, you'll have to trace that wire back until you find it, but from what I see on the schematic, you're right, your problem should be from that splice to the non working side, hopefully that splice is where the the harness splits, but Pete has been known to splice things in strange places. Just keep an eye out for any damage and pinches in the loom as your looking that might give you a tell tale sign of an issue.