Not understanding what you're saying there. Do you mean it has 0V sometimes, and 9V at other times? Are you checking pin#4 of the solenoid for that voltage?
Pin # 3 of the brake switch carries the input voltage from the fuse box, and pin # 1 has the voltage going out to the solenoid when the brake pedal is depressed, and it might be helpful to check what both of those voltages are. Another good thing to check is what's at pin #2 of the solenoid.