I can source the plugs but would rather purchase a new harness as the existing wires look pretty tired from the heat. Does anyone know where I can get a new fuel injector harness?

New probably not going to happen unless you come across some NOS that somebody has.
Also wiring harnesses get a little complicated in the parts catalogs. At least to me they do.
Seems for your '99 it looks pretty specific.
Do you have an automatic or a stick?
Is it 4WD?
Is it a 231 or 242 transfer case?
Now if I had to guess this is the one you want.
56009 776AD
Looks like one from a '98 might work too.
'97 not looking like it and thinking not '00 or '01 either.

Boneyard or Ebay is probably your only option.
There are 2 "main" harnesses in the engine compartment.
The one with the fuel injector connectors you want has 2 connectors to the PCM.
It is not overly hard to pull one from a boneyard Jeep.
I grabbed one from an '00 once. It was on the ground and getting at the CPS was a little difficult.
But then again I didn't just snip things to get it out.
One of the guys at the boneyard said it is taking me so long cause I am removing it like a mechanic and not a boneyard guy. LOL.