line to a relay that connects/interrupts the signal from the controller to the solenoid. You wouldn't want to use the OEM boost solenoid wire as ground to your controller, the controller would be turning off and on all time.. off when you weren't in boost and turning on around 6psi, which is where the OEM system opens up the boost solenoid for higher boost.
Enthusiasts soon understand each other. --W. Irving. Are you an enthusiast?
If you are out to describe the truth, leave elegance to the tailor. Albert Einstein