Sir,

I work as a consultant. I work for a consulting company which is Based in NJ. I don't have a Primary residence. I move project to project in different City/State . In Year 2006 I spend my first 3 months in CA and rest in WI working for clients.
Both in CA and WI I rented an Apartment. But in WI I also registered my Car (Wisconsin plate, since my Florida registration was about to expire). Same for my driver's license too. My kid was also born here.
My company takes the NJ tax out of my wages plus NJ SUI/SDI. Even though I never lived in NJ. I get per-diem per month which is based on the city which is currently in WI. I don't pay tax on that.
Please guide me which state tax should I pay. If I pay WI tax should I also pay tax on the per-diem I got staying in WI. How do I get the refund of the taxes + SUI/SDI from NJ.

Please guide me.

Because you get paid per diem, you can make the argument that you are home-stationed in NJ (even though you may have never been there).

You have to pay state taxes to someone! I suspect that Wisconsin offers the lowest tax rate, but they may want to tax the per diem.

You may want to get professional help, preferably in Wisconsin.