Yes, the modules need to be programmed through forscan and is covered in the guide in beginning of thread. It took only 5-10 minutes to do both modules.
You only need to replace for 15-17, the Steering Control module (clock springs) that came in the 15-17 cars are not compatible with the digital dash that didn't exist at that time.
few different splice methods but I would use simple lap splice and solder and each wire and cover with heat shrink, then wrap entire bundle with cloth electrical tape for oem look. Picture below gives idea of simple lap splice and 2nd pic gives you example of offsetting the wires.
Sorry, not sure how deep this harness goes but I suspect it may be part of the main cabin harness. I will say this as I work for cable and harness manufacturer. There is nothing wrong with splicing when done correctly. Splicing is IPC approved and believe it not cable/wire manufacturers...
Check this thread, it mentions a setting as possible solution Post#208
https://www.mustang6g.com/forums/threads/install-pp-gauges-on-non-pp-model-recap.99467/page-14#post-2569214
Nothing changed since what I told you in post#517. In regards to oil sending unit, I didn't have to change it on my ecoboost but perhaps its different on GT's but I don't believe so. I believe that was theory floating around to try instead of a custom tune that never panned out. You will need...
You will need an n-gauge or similar programmer and have custom tune made to get oil pressure gauge to work, the only other option is if you can find performance shop that can modify the pcm directly(Someone in this thread went this route) and make a one time change. It can't be done via forscan.