That's the one that goes to the break booster. You can buy the replacement one from Ford or switch over to the manual setup like the guys that run the Boss Intake do with their autos thus eliminating it all together.
2015 Challenger Scat Pack Automatic
2015 Mustang GT Automatic 401A Premium SOLD
2011 Mustang GT Automatic 401A Premium SOLD
2003 Mach1 718 rwhp 735 rwtq @ 25 lbs. SOLD
2.3 polished Whipple and every other mod possible