Legal Question in Constitutional Law in Michigan

The Constitution doesn't forbid all warrantless seizures. It only forbids unreasonable warrantless seizures. The type of seizure you describe won't be reasonable all the time, but it will be some of the time.

A request from the landowner is not the only reason police might seize a vehicle on private property, and there are many circumstances which would allow the police onto the property without the owner's permission. That's especially true where the property is open to the public -- as is true of many parking lots, for example.

The police probably won't know or care if your unregistered car is sitting in a friend's back yard. But if it's in a shopping mall's parking lot, there's a good chance that they will.

Even if we assume that the police have no right to enter the property, the owner's rights are the ones being violated. Since the land isn't yours, you have no right to complain about unauthorized access. You have no authority to assert the owner's rights either. So even if the seizure violates his rights, that does not mean it would violate yours.

I don't know whether Michigan law says police may seize a legally-parked car just because it has expired plates. That seems a bit drastic to me. I can think of grounds to challenge such a law. But the ground you mention is not one of them.