Guess a follow up question that needs to be figured out is what lower harness are you using? The KA or the RB one? Not sure how to figure out other than taking pics maybe someone has an idea of what theyre looking for. KA lower harness could be causing the issue youre having w the speedo
Bottom of page 3 and midway down page 4. There are pictures and schematics. Were it my car, i'd pull the ecu and disconnect the body to ecu harness and have a look. If it's crimp spliced or half assed down there i would pull the harness and go through it against the wiring diagrams there and solder/heat shrink everything properly.