2 things:
1) I work in C#, so I may be wrong about this, but I think you have to get the component and THEN get the variable. For example:
var otherScript: OtherScript = GetComponent("AI_CAR");
var newPosition = otherScript.mirrorPos;
2) I think it's best practice to make a temporary variable and then access it. So in the above example, I would then change mirror.transform.position like this:
mirror.transform.position = newPosition;
Obviously it's not not always great to work in vars (sometimes it is, that's an entirely different conversation) but this is just a simple pseudocode example. Hope this helps!
EDIT: here are the docs
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