Hello everyone.
I have the *awesome* habit of custom-installing (half-assing) everything I do on my car, and I usually end up going back after my OCD gets the best of me and doing it all over again very tediously. I figured I'd share my story on installing my 4-point corbeau harnesses, both to help anyone else planning on doing it, as well as ask for some advice regarding it in the end.
First off, the harnesses are great

I do some novice auto cross racing and hit up neighboring road courses wherever the army sends me around the United States. And those things really plant you in the seat, since the stock GT seats are good, but you still slide around with the regular belt and your legs get tired after a few laps of bracing yourself on the turns.
Installing them was a pain in the butt, since I don't have a roll cage or a harness bar or anything of that sort. The provided instructions say you bolt them into where the rear seat belts go, but unless you want to do a complete rear seat delete, it’s impractical, seeing as the provided bolts and nuts in the box don't fit anything the mustang has to offer, and the angles of the bolts and holes in the back just don't work out. So, I bolted them directly to the floor for the back 2 and the side ones go where the stock seat belt goes. This is a much easier task if you just flat out unbolt the seat, install it, and bolt the seat back in (watch to make sure you don't rip out any of the wiring harnesses underneath the seat), after a hour or so of crawling underneath and around my seats, I just said "forget this" and took the whole seat out. MUCH easier.
For a while I just had the harnesses installed in addition to the regular seat belts, which wouldn't stop chiming, until I buckled the belt behind the seat. eventually I went and got 2 of Ford’s free seat belt extenders for mustangs....even though I don't know how anyone who needs one could even FIT in a mustang...cut the tip out, and clipped just the tip in. This is all before I learned you could just disable the belt-minder with a 7 step process found in the owner’s manual...*sigh*
The side of the harness that bolts where the female end of the stock seat belt is (the side closest to the center console) is a tight fit, the metal sizing piece for adjusting the harness sort of gets jammed in between the center console and the stock seat belt clip, and scratches everything whenever you adjust/recline the seat, or wiggle the harness or anything. So I went *back* in there and just took out the old seat belt receiving end-pieces. This both made more room for the harness end-pieces, they fit perfectly now. Nothing scratches, everything bolts in perfectly
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However, the stock seat belt end pieces are wired (surprise!) to the car. 2 wires, just to (I’m assuming) pass a current through when the seat belt is buckled, thus letting the car know your seat belt is on (or not on). Though the chime is no longer a problem...now the air bags are disabled... I tried unplugging the cables, and wiring in a jumper cable between the 2 wires, thus completing the circuit and fooling the car into thinking my seat belt is on...didn’t work.
So now is where I could use any advice anyone has to offer. For now I just re-plugged in the old seat belts with the extender tips, and zip-tied those underneath the seats, so they're out of the way yet still activate the airbags. It works fine, it’s not noticeable, but my obsessive compulsiveness is driving me to find some way to fix this properly. I’d appreciate any advice/tips some of you more experienced guys (or gals) have to offer.
Sorry if this was a boring read, if anyone wants any more information, just let me know. (I can email photos too)