I'm also looking for this particular item (the short cable/sub-harness that goes between the main engine bay harness and the park/turn signal socket). After a lot of time searching, I've concluded that it likely does not exist as a standalone orderable part for MY 16-19. The
other threads I found had people making their own.
Ford sells the part for MY 13-15 (DB5Z-13A006-A), but it appears that they neglected to do so for the redesign from MY 2016 onwards. Thus, those of us with 2016-2019 FPIUs are out of luck.
I consider myself to be pretty adept at searching for Ford parts. I can normally navigate the various catalog websites, browse through the diagrams, and use keyword searches to find what I need. Worst case scenario Google normally gets me unstuck. Here, though, I've come up completely empty. None of my tricks can find the part number for this piece. I think that Ford might sell them included in the engine bay harness, which costs many hundreds of dollars and thus does not make sense to buy.
The park/turn signal bulb socket is a common size, and the socket connector is one Ford used for decades. Pigtails are widely available and inexpensive. While I'd like to avoid hack jobs when I can, I'll likely just get one of those and splice it directly on to the engine bay harness given the lack of better options.
Unlike all the other threads on this issue, my 18 FPIU shipped with the halogen park/turn lamps, not the troublesome Whelen park/turn/warn combo LED units. However, while poking around in the engine bay for unrelated reasons, I discovered that the wiring to my right park/turn socket was falling apart. On both ends, the insulation was chipping off of the wires. Luckily, the damage was contained just to the cable. There weren't any other failing wires anywhere else in the engine bay that I could see. The signals were still working, so I luckily caught this issue in time before failure.
The harnesses had what looks to be a part number label on it, but mine were faded into oblivion on both sides. Can someone with a 16-19 PIU and factory halogen park/turn (no Whelen park/turn/wigwag) crack the hood and look at the labels on the cables leading to the park/turn sockets?
Per the wiring diagram book, this part is specific to the police variant. Normal 16-19 Explorers just connect the headlamps directly to the engine bay harness; there is no intermediate connector. In the diagram below, it's the segments between C126 and C156 (left) & C145 and C1042 (right).
Here are some pictures of my damaged part:
