I do not get the 28v you get when your keys in. Mines steady and varies by the day (which is weird). Minimum voltage in the high 1’s and max voltage in the low 4’s. Not sniffing 28 let alone 12
How many ground wires do you have under the ground bolt thats on the front left of the engine (near the stator cover bolts)? I sold my 99 GTX so I can't be sure, but I think there are 3 grounds wires there. Be sure that those connections are clean and no paint under the wires.
I'm a dope..lol! just remembered my buddy has a 951 gtx in my back he's gonna fix to sell, i just checked and it's a 99 too. It looks to have two wires (not 3), just as you mentioned.