You will need to file a WI resident state income tax return to report all income from all sources (including IL sourced wages).
However, in
order to get the full refund of your IL state income tax withholdings, you will
need to file a nonresident IL return but report zero "0" income from
IL (even though you do have IL wage income). You need to file your IL return
this way because WI does not allow a credit for tax paid to a reciprocal state
on wages, salaries and commissions. If tax was withheld by a reciprocal state
(IL), you must file directly with the state for a refund of those taxes. You
must mail in this IL nonresident state income tax return and include your state
W-2 information. You will want to include an explanatory statement with your IL
return stating your situation (that your IL employer withheld IL taxes from
your wages in error due to a reciprocal agreement with WI).
