My friend is doing a 99 GSR swap with 99 GSR ecu, can we use a 99-00 Civic SI wire harness? I know that it's better to use a 96-98 Civic Ex harness cuz they're both obd2a.

 

If not can I use a 96-98 civic ex automatic harness, if so all i need to do is
change the IAC right?

 

It all depends on what car it is going into. If you are putting the GSR motor into a 99-2000 Civic DX/EX/LX/CX/Si, you should use the 99-00 Si harness, add the iab, change the IAT wiring, and then use an ecu adapter to run the 99 GSR ecu.

If the car is a 96-98 Civic DX/EX/LX/CX, use the 96-98 Civic EX harness, add the IAB, change the CKF, IAT, IACV, and distributor wiring, and you should be good to go.

I may have forgotten a few minor details, but you get the point. Do not try to use a 99-00 Si harness in a 96-98 Civic, or vice versa. The 99+ has one of the ecu plugs in the body harness, whereas the 96-98 have them all in the engine harness.