From the pics, it looks like something is missing.
You’re looking for the female version of the F3 plug on the chassis side.
Then you’ll need to source an actual F3 male plug from a spare harness, partout, eBay, WS, etc.
Find the s15 dash plug pinout, s14 dash plug pinout, repin or swap wires and solder it and it should be good to go.
If you want to make your car run right now, you can hardwire it together without the plug, but it’ll make the harness permanent to the chassis without cutting it.