Replacing a wiring harness for the car won't have ANY effect unless it's damaged, that's like replacing the car for an easy repair.
You could START with a fresh set of STOCK spark plugs (Or AR104 autolites for a cheap set that works well, but DOESN'T last the 100k stock plats. can do).
The boots/connectors on the coils can be replaced, occasionally helps if damaged/old/oil soaked from leaks.
Any maint. done since new?
The IAC is the cylinder underneath the throttle cable in the picture just posted that has a wire connector to it, that round thing with a black top.
Start a fresh thread describing your issues for better answers, and include mileage on the car.