i assume that i am not alone in opting out of paying $10k more for the WRX STI and got the standard WRX instead. Unfortunately, anything and everything online regarding upgrading or modifying the WRX is in terms of the STI model, so when i went to install my Pioneer AVIC-Z2 In-Dash DVD/GPS Navigation System, any vehicle specific installation instructions or diagrams were non existent. The reason this would have been helpful is that the unit, like other similar systems, requires not only a standard wiring harness to interface the head unit to the vehicle, but also requires to be connected to the emergency parking break signal, reverse signal, and vehicle speed sensor (VSS).
Being that i intended to hack the system in order to watch DVDs while the car is not parked, i followed the directions to bypass the built-in security features. The first step is to ground the parking break signal wire, and then ground the bottom right most pin of connector 2 which will keep the unit unlocked when you start moving. This resolves the parking break signal issue, but we still have 2 to go.
The reverse signal is used to engage the backup camera (if you have one installed), and may or may not be used with the GPS functionality, i don't know. After some searching i found the vehicles repair manual where chapter 7 section 23 discusses the back-up light system's wiring schematic which can be found here (Warning: PDF). The reverse signal goes over the Brown/Yellow wire (the same for all models) and can be found under the passenger side kick-plate.
The last signal which the unit requires is the vehicle speed sensor. Note: Even if the unit is bypassed and the VSS signal wont disable the unit, the signal is still important for the GPS functionality as it is not capable of solely running on the GPS signal for vehicle location. My experience shows that without the VSS connected, the unit may assume your driving 65MPH down the highway... backwards. Chapter 7 section 17 of the vehicles repair manual discusses the 3 WRX model's engine electrical systems and their various ECUs can be found here (Warning: PDF). To cover all 3 WRX models: the VSS signal is the Green/Yellow wire connected to pin 1 of connector B134 of the turbo model's ECU (PDF page 4), the Green/Yellow wire connected to pin 10 of connector B147 of the SOHC model's ECU (PDF page 18), and the Green/Yellow wire connected to pin 27 of connector B135 of the STI model's ECU (PDF page 30).