Got the big plug that is attached to the car and the small plug good to go but I don’t have a third big plug on the integra harness to match this one on the eg.

Anyone know what that plug does and if I need to buy a new harness?

 

You will likely have to research all the plugs since you are mixing two different vehicle harnesses. You may very well have to depin and repin plugs to make it all work together. Just because a plug end fits another plug doesn't mean the wires are the same purpose between car harnesses

 

My guess:

It's 10-pin connector C102 of the main wire harness. C102 couples with C213 of engine wire harness that carries many sensor wires.

 

NO DETAILS ON CAR OR HARNESS......