As a Wisconsin resident, ALL your income is taxable by Wisconsin, regardless of where you earn it, unless it is specifically exempted (such as Social Security benefits). Therefore your rental income is taxable by Wisconsin.
You should file an amended Wisconsin return for both 2015 and 2016. You'll need the 2015 and 2016 CD/Download versions of TurboTax, respectively, to do this.
Wisconsin charges 12% interest per year from the due date of the original return on any additional taxes due on an amended return.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
