Mustang 2000 Unknown Part

From the Ford service manual and wiring diagrams it's Connector C155 that connects to the Air Injection Reaction (AIR) relay. The wiring diagrams show a white wire and a black wire junction'ed near C155 going to AIR pump (the pump is also called the SMOG pump).

Now that you have that information I'm not so sure what use it is. It seems to me that you need replacement parts. If this were my project I would check out car-part.com to search for a local salvage yard with the part. When you go to buy the part ask for the car's wiring harness to be included with the AIR relay. Ask for the pigtail to be as long as possible. This should give enough parts to perform a complete repair of connector C155.