Help us help you better by creating a signature that includes your E53 X5's model/model year/build date. This info helps us to give you specific info/suggestions etc that is for your model. There are many changes through the production years, and slight difference between some of the model/engine/tranny configurations that were used on the e53. You may ask a question that deals with something that is slightly different and by not having any model information...you may receive incorrect information.
According to the chart below...it is located on the LEFT side of the vehicle below the dash & nearer to the exterior of the car (not the near the center of the dash):