I've been following your guide but I've run into an issue.
I spliced into the wires behind the head unit but once hooked up to the amp, the radio would not turn on and the car would not start. I checked the connections and it turns out that I have different colored wires running to my pinouts than you did. I thought connecting everything to the correct pins would fix my problem, however it has not. I have RR+ as pin 1, RR- as pin 5, RL+ as 4 and RL- as 8. When I remove one of the spliced connections it's like it resets something and the radio will turn on. I have not tried plugging into the amp with only three inputs, so I do not know if the issue will reappear once connected.
Does anyone have an idea of what my issue could be? I have a RCD510 premium 8 radio, the PBR300X1 and my sub has not arrived yet, so the output from the amp is not connected to anything.