It seems that for different years and truck configurations the location is different. My '04 Reg Cab Long Bed has it located where SparksTech described - between the Cab and Bed above and outboard from the gastank. I can actually see mine if I look down between the cab and the bed out toward the drivers side. I think its easier to get at if its behind the gas tank (from looking anyways).
If you have the replacement part it might not look exactly the same - since they are using and upgraded part now.