@fd3s7007: I want to learn a bit more of the wiring on 99' cars because so far there are just a few details I found on the internet.
@cjr: Thanks. It's all clear now. I want to make a wiring extension connector harness for my ecu so that in the future I don't have to splice any wires on the original connector harness if the installation procedure requires me to do so. That's why I need to confirm about the empty slot.
like cjr said the ECU only have 3 connectors with each pins are 34p, 22p and 28p.