I'm swapping out my old 94 ex z6 for a z6 off a 94 si hatch and when it came to the engine harness it won't fit. As much as I've researched, everyone is saying that ex/si is the same, which makes sense since they both have a d16z6. But the plugs off the si engine harness has plastic pieces around the edges that doesn't let it plug at all. The third smaller plug fits fine.

 

Well that's embarrassing, right after posting this thread I decided to try to pull out the plastic piece and it turns out that there's 2 of them attached together. Mystery solved.