I am currently renting and working in the state of Illinois. However, I was thinking about buying a car and having my dad who lives in Michigan cosign for it. I still consider my "permanent address" to my parents place in Michigan. Is it legal for me to register my car in Michigan? Would the answer change if I could make my dad a co-owner on the car?
The question of where you are a resident is supposed to be a matter of objective fact rather than subjective viewpoint, but the law (in Illinois) does not give a uniform legal definition of residency. For instance if you have an out of state driver's license, you are not a resident, for tuition purposes. If you are in Illinois "for other than a temporary or transitory purpose during the taxable year", you are a resident for tax purposes, which includes being "employed in a position that may last permanently or indefinitely". If you are registered to vote in Illinois, you are a resident. The matter of residency has presumably been established in connection with employment and state income tax. As a resident, you are required to transfer your vehicle registration to Illinois (625 ILCS 5/3-101(a))
Except as provided in Section 3-102, every owner of a vehicle which is in this State and for which no certificate of title has been issued by the Secretary of State shall make application to the Secretary of State for a certificate of title of the vehicle.
However, if the vehicle is owned by a non-resident because of 3-102 which says that
No certificate of title need be obtained for...(3) a vehicle owned by a non-resident of this State and not required by law to be registered in this State
If you co-own the car, this raises the question whether the fact that it is also owned by a non-resident means that the vehicle need not be registered in Illinois (as long as it is registered in Michigan: see the reciprocity section (625 ILCS 5/3-402(B))). The statute does not directly address the matter of co-ownership where the owners are residence of different states, but under a literal reading of §102 the fact of it being registered to a Michigan co-owner should suffice to be in compliance with the law. Naturally, your (Illinois) attorney should inform you as to what the law is.