CPP is clutch pedal position. Your PCM is looking for a switch that should not exist on your car.
Looking at PPT B in 2006 PC/ED shows that if you have P0689, you should jump straight to verifying voltage on the ISP-R circuit. Since you are having starting issues, I would assume there is actually a problem with the ISP-R circuit that you haven't been able to isolate with static testing. Although the static voltage test seems to be fine, I would load test it and carry out a voltage drop test as well. Load testing is a dynamic test and will actually work the circuit. Ideally, we want to see less than 0.3V dropping across the circuit.
With that being said, I'm looking at an 05 wiring diagram and I can't find ISP-R either. It looks like the control side of PCM power relay is constantly grounded and receives hot in run or start power from the SJB when the ignition is cycled. Your wiring diagrams are for a 2006 and are very different from the 2005 wiring diagrams I pulled from PTS. ISP-R seems to only be present on 2006/2007+ cars.