I have a 1999 Mustang GT, i just ordered a SCT X Cal II from brenspeed and they need my processign code in order to go the tuens, where is it located on my car, someone told me the passener side door, but can you please give me an exact location

 

It's behind the kick panel on the passenger side. Pull it off, and there will be a sticker on your computer that hasbarcodes and numbers on it. You're looking for the one that has 3 letters and a number. (Mine is RKN2.)