@coolbean wrote: "I would still need a tax credit from CA."
Did you have Illinois-source income after you moved to California? If you did not, then you are not due any tax credit from CA. CA will only grant a credit if you have income that is legitimately taxed by both CA and another state.
If your employer mistakenly withheld IL taxes after you became a CA resident even though you no longer lived or worked in Illinois, then no credit is due. Your only remedy is to obtain a refund of the incorrectly withheld taxes from Illinois. CA will not give you a tax credit for a mistake made by your employer.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
