So much drama...
Contact the buyer. Indicate that he is in arrears on the storage you agreed to. Inquire what their intentions are, and make a decision on next steps based on the response or lack of one within a defined time period.
Anything else is speculation and unethical.
In Minnesota, you if you want the car gone, since you do not have title, you would need to contact law enforcement, and the car would be impounded. A notification period follows, where the legal owner can reclaim the property. If the owner does not reclaim, the impound lot or goverment authority who impounded the vehicle (not the private property owner) sells the car, and you would have the opportunity to buy it.
No idea what regulations are where the OP is.