You need insurance from Ontario.
To obtain insurance from Michigan, you would need a Michigan residence and driver's licence, and you'd have to get your car plated and registered in Michigan.
Now the long version: Your insurance company has to provide insurance at the location where the car is based, so to speak. Obtaining out-of-province insurance for a car registered and plated in Ontario is illegal, even if the car spends most of its time out-of-province. Example: If you live in Windsor, Sault Ste. Marie or Sarnia, you'd still need Ontario insurance for your vehicle even if you work in Detroit, American Sault or Port Huron. I used to cross-border commute (lived in Windsor, worked in Detroit) and my insurance had to be from Ontario. Of course, if you live in Ontario, your insurance is valid for trips outside the province (subject to its terms and conditions); likewise, if you lived in Michigan, it would be valid in Ontario... but you'd need to be a bona-fide Michigan resident.
Some people have attempted to save money by deliberately misreporting where the car is based "in province." A while ago, some Toronto residents who had cottages in places like Muskoka and Prince Edward County were falsely using their "cottage address" as their official home residence to obtain cheaper insurance. That practice came to a screeching halt after insurance companies investigated when a surprising number of Muskoka and Prince Edward County "residents" got into crashes in Toronto. Some of the worst offenders got convicted of insurance fraud.