1990 IH 4600 replacement wiring harness

I do not know of aftermarket companies that make vintage wiring harnesses. I think your best bet is to call your nearest International dealer, give them the vin number of the truck and ask them if one can be ordered. International does not have stock on any complete harnesses, but they are available and they have to be made when ordered to how the truck was built originally. Since this over 30 years old, many of the connectors are no longer used and so getting a harness made may not be possible for something of this age, but it is still worth a try. Getting a harness ordered and made will take several weeks if not a few months depending on what is available. Good luck.