the small hose on the plenum, thats where the pressure comes from to open the waste gates, its also with the vaccumm lines that go to the factory solenoid
the way the factory solenoid works that it release the pressure from the waste gates by bleeding the air back into the intake pipes of one of the turbos
so if you are going to have a after market EBC get the pressure from that small hose at the back of the plenum then the line that goes to the wastegates use the the line that was on the standard solenoid. (make sure its the line that goes to the waste gates) the other line of the standard solenoid just goes back to the intake pipe so that doesnt need to be used anymore.
just make sure when take off the hose off the plenum to put the new hose on for the EBC solenoid, block the hose you took off the plenum soo you dont have pressure leak to the wastegates
another way to find out where the lines go to, is to blow air through them, if you have a air compressor
hope it helps