That's either your MAP OR BAP sensor depending on the year of your Mustang, 87-88 uses a MAP sensor.
The only difference is in the S/D the vacuum line is hooked to the intake to find manifold pressure; on a BAP it is unhooked to find barometric pressure.
You posted MAP sensor so I'll assume yours is S/D .
While looking MAP sensor on the firewall on the left side of MAP sensor you'll see a round shaped fitting this is where vacuum hose connect too. The other end of the hose connects to the bottom of the upper intake to a round metal fitting.