Marge: The national grammar rodeo? I wish I were going. Oh, wait, wait; I wish I was going. Is that right, Bart?
Bart: I dunno.
So, which is it?
I think were is correct in this case.
You have entered the dark underworld of the subjunctive mood. My understanding is that, in English, the subjunctive is reserved for contrary-to-fact statements of desire—in this case, a wish. Marge is wishing for something that will not happen, so she should use the past subjunctive: “I wish I were going.”
There are three other moods in English: the indicative, imperative, and conditional. But I don’t understand the nuances of each well enough to explain them.